A method of controlling a drive train comprising an electric motor (12), a gearbox (1) and an internal combustion engine (13). The gearbox (1) is placed between the electric motor (12) and the internal combustion engine (13) in the drive train. The speed of the electric motor (12) is synchronized to the speed of the internal combustion engine (13) in order to shift the gearbox. The gearbox (1) is shifted between different operation modes by means of controlling the position of a dog clutch (6).