Thank you, Gmayor! I'll keep your code for more complex strings. Actually my case is not so complex, so I've rewritten my code as follows (thank you, Vivka!!):
Code:
Sub LastNameRN()
Set oRng = ActiveDocument.Paragraphs(1).Range
If InStr(1, oRng, ",") > 0 Then
oRng.Collapse
oRng.MoveStartUntil Cset:=","
oRng.Previous(unit:=wdWord, Count:=1).Select
Else
oRng.Words.Last.Previous(unit:=wdWord, Count:=1).Select
End If
End Sub