A method of coupling a combustion engine of a parallel-hybrid drive train via a clutch positioned between the engine and an electric machine with the clutch device initially disengaged, the engine rotational speed approximately at an engine idling rotational speed, and the machine rotational speed larger than the engine idling rotation speed. When a request is made for a drive torque value which is larger than a torque threshold, the engine rotational speed is brought to a rotational speed value above the machine rotational speed and the clutch is slipping engaged once the rotational speed value is reached, while the engine, when the drive train decelerates and a request is made to couple the engine, is brought into deceleration cancellation and the engine rotational speed is brought up to the machine rotational speed while engaging the clutch and dependent on a transfer ability of the clutch.