function countChar(textarea,e,spanName)
        { 
            var length =140 - textarea.value.length;
            
            if(length == 0 || length > 0)
            {
                document.getElementById(spanName).innerHTML = length;
            }
            
            if(length == 0)
            {
                if(checkSpecialKeys(e))
                {
                    
                     if(window.event)//IE
                      e.returnValue = true;
                }
                else
                {  
                    if(window.event)//IE
                      e.returnValue = false;
                    else//Firefox
                        e.preventDefault();
                }
            }
            
            if(length < 0)
            {                           
                textarea.value = textarea.value.substring(0,140);
                document.getElementById(spanName).innerHTML = '0';
            }
        } 
        
        function checkSpecialKeys(e)
        {
            if(e.keyCode == 8 || e.keyCode == 46)// && e.keyCode!=37 && e.keyCode!=38 && e.keyCode!=39 && e.keyCode!=40)
                return true;
            else
                return false;
        }  

<div class="textareaBox"> 
                           <asp:TextBox runat="server" ID="txt_content"  MaxLength="140" TextMode="MultiLine"></asp:TextBox>
                       </div>  
                       <div class="sendMsg">您可以输入 <label id="counter">140</label> 字  <asp:Button ID="Btn_Save" runat="server" OnClientClick="return checkstr();" Text="提 交" CssClass="btn_green" OnClick="Btn_Save_Click"  /></div>

本文转自94cool博客园博客，原文链接：http://www.cnblogs.com/94cool/archive/2011/07/04/2097683.html，如需转载请自行联系原作者