When peter pettigrew turn into a rat, his clothes fall off because of his shrunk size...but when he turn into himself again, his clothe automatically comes back in his body...the same goes for sirius black and lupin too...how's that possible??

It's a children's series, so the magic prevents nudity.

Good question! A mistake, I guess - in all cases their clothes should disappear with them when they transform.

well harry potter is a magic based movie r8....... so assume that sudden appearance of the dress is due to magic... :P

So it was not a Rated R movie. oder he transformed like the werewolf perfessor

I would assume Sirius and Peter are skilled enough at magic to materialize clothes onto their bodies as they transform back.

Lupin is different since he's a werewolf and essentially has no control over it so he should be naked when he turns human again, but.. did we ever see him transform back into human form? I don't remember that.