Rob Kardashian is in ''beast mode''.

The 'Keeping Up with the Kardashians' star is working hard with personal trainer GUNNAR PETERSON and is determined to tackle his recent weight gain.

Gunnar said: ''I think Rob knows exactly what to do. He's worked out [before] and he's focused.

''We're doing intervals with strength movements: squats, the sled, swings and push presses. It's about getting his heart rate up and keeping it up.

''I'm a firm believer in trying to make something that might not be considered fun by a lot of people, fun. And hopefully that makes the person have a positive association with it and they want to come back.

''Rob works out in beast mode. When I see him, we laugh, have fun and he gets after it hard at the gym. I know he can do this.''

The pair will continue to work together until the 27-year-old sock designer - who reportedly missed his sister Kim Kardashian's wedding to Kanye West because he was so unhappy with his appearance - is happy with his physique.

Gunnar added to People magazine: I don't have a scale, I'm not big on measurements. Until a person looks in the mirror and is happy with themselves, you have not reached your fitness goal.''