In A1:

="X_XX_"&TEXT(ROW(),"000")&"_XX"

Copy down as far as needed

--
Kind regards,

Niek Otten
Microsoft MVP - Excel

"kkr" wrote in message ...
|I am using excel Sheet which contains around 1000 cells to be numbered
| serially,
| eg:A1 as X_XX_001_XX
| A2 as X_XX_002_XXX
| A3 as X_XX_003_XXXX ...
| can anyone let me know solution for this...