Thanks for your quick reply, but I after I traced the codes.. I think the result value will be rounded when it execute the format command:

Msgbox mVATValue ' Result = 1658.97138 (This is the correct result)

' The value will be rounded when below code run..

mDisplay = Format(mVATValue, "###,###,##0.0000")

mDisplay will shown 1,658.9714

and also I have followed your suggestion to change all of Single data type to Double data type.. but the same problem is still occurs.. any idea? Thanks

Regards
Winan