Hi Wongchaa,
This worked for me :
If you're trying to put the up/down arrow in cell A1 (based on the value of B1).
Format A1 with text in Wingdings.
In cell A2 put the character for up-arrow (you'll find it by Insert Symbol & then looking up the symbol)
In cell A3 put the character for down-arrow (you'll find it by Insert Symbol & then looking up the symbol)
Put the formula in cell A1 that tells whether you want to use up or down eg:
IF(B1>0,$A$2,IF(B1=0,"",$A$3))
For extra visibility, I also use conditional formatting to display the symbol in green when it is the up arrow, & red when it is the down arrow.