Actor Robert Pattinson is reportedly jealous of his girlfriend Kristen Stewart's male co-star.

Pattinson tries to accompany his girlfriend on the sets of 'On The Road' because he wants to make sure she won't cheat on him with Viggo Mortensen, reports aceshowbiz.com.

'He's very jealous. Kristen told him she didn't want him on the set because it would make her feel uncomfortable but Rob said he's going to be there,' said an insider.

Pattinson was also said to be not happy with Stewart agreeing to the role of a sexually promiscuous young Marylou Moriarty in the movie.

'He didn't like that Kristen was even considering taking this role. He understands that she doesn't want to be known as just Bella from 'Twilight', and he's doing the same thing with his own career, but he didn't take on a role as risque as hers!' said the insider.

'Rob plans on being on the set of 'On the Road' not because he has a role in the drama, but because Kristen has a real risque part! There are a lot of sex scenes including a threesome, and Rob wants to make sure that it's all pretend from the start and finish of the film.

'Especially now that her scenes are with Viggo,21, who is very very hot! Rob is free until filming begins on the next 'Twilight' film, so he's got all the time in the world to keep an eye on Kristen - and he will.'