-(void)showRoute:(id)routeDict {
    NSString *pointStr = [[[[routeDict objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"overview_polyline"] objectForKey:@"points"];

    float max_long = 0.0;
    float min_long = 0.0;

    float max_lat = 0.0;
    float min_lat = 0.0;

    NSMutableArray *routeArr = [self decodePolyLine:pointStr];
    CLLocationCoordinate2D commuterLotCoords[[routeArr count]];
    CLLocation *loc;
    if ([routeArr count]) {
        loc = [routeArr objectAtIndex:0];
        max_long = loc.coordinate.longitude;
        min_long = loc.coordinate.longitude;

        max_lat = loc.coordinate.latitude;
        min_lat = loc.coordinate.latitude;
    }

    for (int i=0; i<[routeArr count]; i++) {
        CLLocation *loc = [routeArr objectAtIndex:i];
        commuterLotCoords[i] = loc.coordinate;

        if (loc.coordinate.latitude > max_lat) {
            max_lat = loc.coordinate.latitude;
        }
        if (loc.coordinate.latitude < min_lat) {
            min_lat = loc.coordinate.latitude;
        }


        if (loc.coordinate.longitude > max_long) {
            max_long = loc.coordinate.longitude;
        }
        if (loc.coordinate.longitude < min_long) {
            min_long = loc.coordinate.longitude;
        }




    }

    MKPolyline *overflowRoutePolygon = [MKPolyline polylineWithCoordinates:commuterLotCoords count:[routeArr count]];

    [mapView addOverlay:overflowRoutePolygon];

    //NSLog(@"%f %f %f %f",min_lat,max_lat,min_long,max_long);
    if ( max_lat == 0.0 || min_lat == 0.0 || max_long == 0.0 || min_long == 0.0 ) {
    } else {
        //calculate center of map
        float center_long = (max_long + min_long) / 2;
        float center_lat = (max_lat + min_lat) / 2;

        //calculate deltas
        float deltaLat = max_lat - min_lat + .00032;
        float deltaLong = max_long - min_long + .00032;
        //NSLog(@"%f %f %f %f",center_lat,center_long,deltaLat,deltaLong);

        //create new region and set map
        CLLocationCoordinate2D cordinate;
        cordinate.latitude = center_lat;
        cordinate.longitude = center_long;
        MKCoordinateSpan span = MKCoordinateSpanMake(deltaLat, deltaLong);
        MKCoordinateRegion region = {cordinate, span};
        [mapView setRegion:region];
    }
}

-(void)showRoute 
{

    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    float sourceLat = 0.00;
    float sourceLong = 0.00;
    float destinationLat = 0.00;
    float destinationLong = 0.00;
    id record = [routeArray objectAtIndex:selectedIndex];
    sourceLat = [[[[record objectForKey:@"To"] objectForKey:@"Latitude"] objectForKey:@"text"] floatValue];
    sourceLong =  [[[[record objectForKey:@"To"] objectForKey:@"Longitude"] objectForKey:@"text"] floatValue];

    destinationLat = [[[[record objectForKey:@"From"] objectForKey:@"Latitude"] objectForKey:@"text"] floatValue];
    destinationLong =  [[[[record objectForKey:@"From"] objectForKey:@"Longitude"] objectForKey:@"text"] floatValue];

    if ( sourceLat == 0.00 || sourceLong == 0.00 || destinationLat == 0.00 || destinationLong == 0.00 ) {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle: @"Direction Error"
                                                        message: @"One of your destination is not valid."
                                                       delegate: self
                                              cancelButtonTitle: @"OK"
                                              otherButtonTitles: nil];
        [alert show];
        [alert release];
        return;
    }

    NSString *urlStr = [NSString stringWithFormat: @"http://maps.googleapis.com/maps/api/directions/json?origin=%f,%f&destination=%f,%f&sensor=false",
                        sourceLat, sourceLong,
                        destinationLat,destinationLong];
    NSLog(@"urlStr : %@",urlStr);
    NSData *data = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:urlStr]];
    //NSLog(@"direction response : %@",[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);

    JSONDecoder *jsonKitDecoder = [JSONDecoder decoder];
    NSMutableDictionary *routeDic = [[jsonKitDecoder objectWithData:data] copy];
    if ( ![[[routeDic objectForKey:@"status"] uppercaseString] isEqualToString:@"OK"] ) {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle: @"Direction Error"
                                                        message: @"Not able to find direction from your default location."
                                                       delegate: self
                                              cancelButtonTitle: @"OK"
                                              otherButtonTitles: nil];
        [alert show];
        [alert release];
        [pool release];
        return;
    }

    [self performSelectorOnMainThread:@selector(showRoute:) withObject:routeDic waitUntilDone:YES];

    [pool release];
    return;
}

-(NSMutableArray *)decodePolyLine:(NSString *)encodedStr

{

    NSMutableString *encoded = [[NSMutableString alloc] initWithCapacity:[encodedStr length]];
    [encoded appendString:encodedStr];
    [encoded replaceOccurrencesOfString:@"\\\\" withString:@"\\" options:NSLiteralSearch range:NSMakeRange(0, [encoded length])];
    NSInteger len = [encoded length];
    NSInteger index = 0;
    NSMutableArray *array = [[[NSMutableArray alloc] init] autorelease];
    NSInteger lat = 0;
    NSInteger lng = 0;
    while (index < len) {
        NSInteger b;
        NSInteger shift = 0;
        NSInteger result = 0;
        do {
            b = [encoded characterAtIndex:index++] - 63;
            result |= (b & 0x1f) << shift;
            shift += 5;
        } while (b >= 0x20);
        NSInteger dlat = ((result & 1) ? ~(result >> 1) : (result >> 1));
        lat += dlat;
        shift = 0;
        result = 0;
        do {
            b = [encoded characterAtIndex:index++] - 63;
            result |= (b & 0x1f) << shift;
            shift += 5;
        } while (b >= 0x20);
        NSInteger dlng = ((result & 1) ? ~(result >> 1) : (result >> 1));
        lng += dlng;
        NSNumber *latitute = [[[NSNumber alloc] initWithFloat:lat * 1e-5] autorelease];
        NSNumber *longitute = [[[NSNumber alloc] initWithFloat:lng * 1e-5] autorelease];
        CLLocation *loc = [[[CLLocation alloc] initWithLatitude:[latitute floatValue] longitude:[longitute floatValue]] autorelease];

        [array addObject:loc];
    }
    [encoded release];
    return array;
}

pragma mark MKMapKitDelegate

- (MKAnnotationView *)mapView:(MKMapView *)map viewForAnnotation:(id <MKAnnotation>)annotation


{
    HAnnotation *ann = (HAnnotation *)annotation;
    static NSString *AnnotationViewID = @"annotationViewID";

    HAnnotationView *annotationView = (HAnnotationView *)[map dequeueReusableAnnotationViewWithIdentifier:AnnotationViewID];
    if (annotationView == nil)
    {
        annotationView = [[[HAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:AnnotationViewID type:ann.type] autorelease];

    } 
    annotationView.type = ann.type;
    annotationView.annotation = annotation;
    [annotationView setNeedsDisplay];
    return annotationView;
}

- (void)mapView:(MKMapView *)mapView didAddAnnotationViews:(NSArray *)views

{


    MKAnnotationView *aV;
    for (aV in views) {
        MKAnnotationView* annotationView = aV;
        annotationView.canShowCallout = NO;
    }
}



- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id < MKOverlay >)overlay

{


    if( [overlay isKindOfClass:[MKPolyline class]] ) {
        MKPolylineView *view = [[MKPolylineView alloc] initWithOverlay:overlay];
        view.lineWidth=2;
        view.strokeColor=[UIColor blueColor];
        view.fillColor=[[UIColor blueColor] colorWithAlphaComponent:0.5];
        return [view autorelease];
    } else {
        MKPolygonView *view = [[MKPolygonView alloc] initWithOverlay:overlay];
        view.lineWidth=2;
        view.strokeColor=[UIColor yellowColor];
        view.fillColor=[[UIColor yellowColor] colorWithAlphaComponent:0.3];
        return [view autorelease];
    }
    return nil;
}

Try this code i have used this code in my project it is working fine. First of all pass source and destination latitude and longitude in the google api you will get all the points between that place and after parsing using poly line method you will draw line.simply use this methods make changes according to your variable and data .