The invention concerns a method and a device for determining the motor moment constant kM of an electric motor by measuring motor parameters on the running motor. For reduction of the previously considerable measuring effort it is proposed that firstly the generator voltage UEMK produced by the motor is measured, and in that the motor moment constant kM is calculated by division of the generator voltage UEMK and the speed of rotation fMot of the motor, taking into consideration at least one further constant. The method and the device are suitable for DC motors and for 3-phase synchronous motors.