尝试一下

Sub test()

Dim Lastrow As Integer
Dim range As range
Dim c As range


With Worksheets("Overdue PO")
Lastrow = Columns("D:F").Cells.Find(What:="*", LookIn:=xlValues,   SearchOrder:=xlByColumns, SearchDirection:=xlPrevious).Row
    .range("D2:F" & Lastrow).Select
    Set range = Selection
End With

For Each c In range
    c.Value = Application.WorksheetFunction.Proper(c.Value)
Next c


End Sub