A method for operating an hybrid power system comprising an engine; a clutch; a transmission having an input shaft and an output shaft and different transmission ratios, wherein the engine is directly connected to the input shaft of the transmission via the clutch; an energy storage device; an electric motor having an output shaft, the motor being connected to the energy storage device; and wherein the output shaft of the transmission is directly coupled with the output shaft of the motor using a spline connection to provide a coupled power output to the common output shaft to avoid interruption when shifting up or shifting down the transmission ratio, the method further comprising the steps of: transferring the power supplied by the output shaft of the transmission and the power supplied by the output shaft of the motor to a common output shaft, and transferring the output power of the common output shaft to the directly coupled wheel of a vehicle, setting the work mode of the hybrid power system to be electric-only power mode or a parallel mode in which both, power from the electric motor and power from the engine contribute to the power which is output to the wheel of the vehicle based on a current vehicle velocity and an electric quantity of the energy storage device; wherein, if the work mode is set to the electric-only power mode, then assigning a power level such that an output power of the motor is based on the demanded power, and wherein the output power of the engine is set to be zero; and characterized in that if the work mode is set to the parallel mode, then assigning a power level by a controller which is adapted to control both, the engine and the electric motor, such that an optimal output power of the engine to achieve the highest possible degree of efficiency and an output power of the motor are both assigned based on a rotational speed of the crankshaft of the engine and this process of assigning power of the engine and the electric motor is repeated as many times as the total power demand which is required from a user changes. transferring the power supplied by the output shaft of the transmission and the power supplied by the output shaft of the motor to a common output shaft, and transferring the output power of the common output shaft to the directly coupled wheel of a vehicle, setting the work mode of the hybrid power system to be electric-only power mode or a parallel mode in which both, power from the electric motor and power from the engine contribute to the power which is output to the wheel of the vehicle based on a current vehicle velocity and an electric quantity of the energy storage device; wherein, if the work mode is set to the electric-only power mode, then assigning a power level such that an output power of the motor is based on the demanded power, and wherein the output power of the engine is set to be zero; and characterized in that if the work mode is set to the parallel mode, then assigning a power level by a controller which is adapted to control both, the engine and the electric motor, such that an optimal output power of the engine to achieve the highest possible degree of efficiency and an output power of the motor are both assigned based on a rotational speed of the crankshaft of the engine and this process of assigning power of the engine and the electric motor is repeated as many times as the total power demand which is required from a user changes.