hi, Nader !

Is it possible to catch the event paste ?
I'd like to inform the user when is doing a copy & paste
that some data needs to be changed once the paste is done.

i think you could start with the following workaround and adapt to meet your conditions:

[ThisWorkbook code module]

Dim Possible_Paste As Boolean
Private Sub Workbook_SheetActivate(ByVal Sh As Object)
Possible_Paste = Application.CutCopyMode = xlCopy
End Sub
Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
Possible_Paste = Application.CutCopyMode = xlCopy
End Sub
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
If Possible_Paste Then
MsgBox "You have modified cell(s) that seems to come from a Paste-operation ..."
End If
End Sub

hth,
hector.