How to: Refer to Sheets by Index Number [Excel 2007 Developer Reference]

An index number is a sequential number assigned to a sheet, based on the position of its sheet tab (counting from the left) among sheets of the same type. The following procedure uses the Worksheets property to activate worksheet one in the active workbook.

Sub FirstOne()
    Worksheets(1).Activate
End Sub

If you want to work with all types of sheets (worksheets, charts, modules, and dialog sheets), use the Sheets property. The following procedure activates sheet four in the workbook.

Sub FourthOne()
    Sheets(4).Activate
End Sub

Note
The index order can change if you move, add, or delete sheets.