A method of controlling a hybrid drive train of a vehicle having, in series, an internal combustion engine, a clutch, an electric motor and a transmission with an output connected to the drive axle. During traction operation, the vehicle changes from an electric driving mode into a combination driving mode or a combustion engine driving mode, in that the clutch is engaged and the combustion engine torque is temporarily increased. The method includes regulating engagement of the clutch at least until reaching a starting rotational speed of the combustion engine such that the acceleration of the combustion engine occurs according to a predetermined progression of rotational speed and that the torque of the combustion engine is increased by the same amount as the transferable torque of the clutch is increased by the engagement process.