Lastrow is an integervalue, so you just calcualte that from the row property, you don't also select (none of the code I have given you selects it). You then pass that variable to the Rows object
[vba]Sub BoldLastRow()
'Bold last row
Dim Lastrow As Long
Lastrow = .Cells(.Rows.Count, "A").End(xlUp).Row
Rows(Lastrow).Font.Bold = True
End Sub[/vba]
BTW, the tags are vba and /vba within brackets