There is described a method for controlling a hybrid electric vehicle power-train comprising an internal combustion engine, an electric machine and a transmission, the transmission having a gear set with three rotational elements rotationally coupled to the internal combustion engine, the electric machine and the vehicle driving wheel, a rotational speed of any one of the three rotational elements being mechanically defined by the rotational velocity of the remainder of the three rotational elements. The method comprises adjusting the torque of the electric machine to change the speed of the internal combustion engine and to control the velocity of the electric machine above a predetermined value, and decreasing the torque of the internal combustion engine while substantially maintaining the intake manifold pressure of the internal combustion engine in response to a decrease in the demanded engine output. The method may advantageously improve both of the engine efficiency and the power transmission efficiency, then may improve the operating efficiency of the overall power-train system having the internal combustion engine and the transmission coupling the engine, the electric machine and the driving wheels.