i just watched new episode too and im confused about one thing..how can demon possessed Bobby?i thought he has tattoo that save him from demon possession

Like LEH said, we don't know for sure if Bobby has a tattoo. But even if he has a tattoo or a charm to protect him- the only thing it protects him from is a demon getting inside him. It doesn't stop the demons from attacking him, taking the charm away or destroying the tattoo so that they can possess him.

Someone I know suggested that because Luficer has been released, the demons now have extra mojo and some of the weaker amulets and anti-possession spells may no longer work. It's a really good question though, hopefully it's addressed on the show at some point. Because Bobby's the last person I see letting himself get possessed. ?:)

We don't actually know if Bobby's got a tatoo, we only know Sam and Dean has, first seen in season 3... Though the boys tatoos were the same "pattern" as the amulets Bobby gave them in BUABS:)
But it is a good question: How could he be possessed? I mean, he was the one who gave the boys the amulets, and he's the most experienced hunter the boys have ever known...
Was he so reckless or did the demons find a way to break all spells? Or my theory, maybe he was outnumbered, I mean, there was Meg, another demon, and possible also the demon who did possessed Bobby, maybe he they found a weak spot, threaten him about the boys or something, to possess him and use him to find the boys??
Don't know, but we can always hope we get an answer in one of the upcoming episodes:)