A method for accelerating a vehicle driving forward, in which the vehicle has a propulsion system including a combustion engine with an output shaft (2a), a gearbox (3) with an input shaft (3a), an electric machine (9) comprising a stator and a rotor, and a planetary gear comprising a sun gear (10), a ring gear (11) and a planet wheel carrier (12). When accelerating the vehicle the torque of the electric machine is controlled and the rotational speed of the combustion engine is controlled until the components of the planetary gear have the same rotational speed and may be interlocked.