if joker is bound by the lasso,will he tell the truth? or can he resist it

Yes he will tell the truth.The lasso also cures insanity.The joker isn't insane though, he just pretends so he can escape the death penalty.

@moywar700: this is old but i'm gonna bump it.

Joker is definitely insane. I don't see anyway around it.

Magic beats insanity. Just because the Joker's insane doesn't mean that he doesn't know the truth or is incapable of telling it.

The whole "Joker is just faking insanity" was just a speculation mentioned in a few non canon event projects like Batman: Black and White and Justice. Martian Manhunter, Zauriel,, The Stranger, The Specter, Raven and other have all been in Joker's mind and soul and confirmed he is hopelessly insane. Not to mention his EKG scan in Joker: The Last Laugh. You can't fake your own brain physiology. Not if your human anyway. Maybe Batman could through force of will or post hypnotic suggestion or something but he would be the only one.

I think Joker would react the same way he did in The Demon Laughs , Rock of Ages, and Going Sane story lines when he went temporarily sane. In Going Sane he forgot about ever being the Joker, fell in love and became a kind, good man. Until Batman showed up alive. Even Batman wasn't sure if the man he was seeing used to be The Joker. ( His skin was made flesh tone by medication he had to take daily) In Rock of Ages Joker seemed very incoherent and confused and in shock. "What have I been doing with my life? I..I think I need help." In The Demon Laughs he even helped Batman stop Ras Al Ghul from committing global genocide, before his insanity returned.

So yes, Wonder Woman's lasso would restore his sanity, and he would be repentant and try to make amends. But it wouldn't have any lasting effects once the lasso came off.