I am using a 16*2 LCD, the LCD works well but the back-light is almost black, which makes the characters on LCD hard to tell. How should I solve this problem?

Thanks!

Are you sure it's a backlight issue? Check you have the suggest current through the LED at pin 15 and 16.

Also did it ever work? If not it may be worth checking the LCD contrast at pin 3, I have found this often causes poor performance and I normally use a variable resistor across gnd and Vcc with the wiper going to pin3 adjusted for best contrast.

Sorry if you have done all this when you say 'works well', but thought I'd ask before you attempt anything more tricky!

Thanks, Kevin!

The characters are printed correctly and that is why i think it is working. However, the background of the characters are almost black, which makes me hard to tell those characters.

I think it is the contrast problem. So I will also try to use a variable resistor across gnd and Vcc with the wiper going to pin3 adjusted for best contrast. Thanks again, Kevin!