there is only 5 bytes in Number.dat: " 1234". FileID=fopen("Number.dat") if FileID <> -1 then tempStr=fgets(FileID, 40) tempNum=val(tempStr) tempStr=padl(allt(str(tempNum+1)), 5, " ") fwrite(FileID, tempStr) && return 0 at this line, i.e. can't write endifI have tried fwrite(FileID, "tempStr") and fwrite(FileID, &tempStr), both not work.