If the question was correct then that should be an AND
and you might have to use SUM if either A1 or B1 is empty

=IF(AND(a1="",B1=""),"",SUM(A1:B1))

sometimes people put a space into a cell or a null string simiar to what is
being entered into Cell C1 with ="", so you might have to consider
=IF(AND(TRIM(A1)="",TRIM(B1)=""),"",SUM(A1:B1))

if the cell is actual empty, tests true for =ISBLANK(A1), then would work for A1+B1
but a cell with a null string would fail with a #VALUE! error..
SUM will ignore text constants.

HTH,
David McRitchie, Microsoft MVP - Excel [site changed Nov. 2001]
My Excel Pages: http://www.mvps.org/dmcritchie/excel/excel.htm
Search Page: http://www.mvps.org/dmcritchie/excel/search.htm

"Chip Pearson" wrote in message ...
Try

=IF(OR(A1="",B1=""),"",A1+B1)


--
Cordially,
Chip Pearson
Microsoft MVP - Excel
Pearson Software Consulting, LLC
www.cpearson.com




"bayanbaru" wrote in
message
...
My formula is C1=SUM(A1: B1). I want to use IF function. If A1
and B1 is
blank C1 should be blank. Right now it show C1=0