Abstract: For parallel hybrid vehicles, a nonlinear observer based novel position servo control algorithm was designed without the need of speed sensors. This algorithm, according to the feedback of system states, can implement high-quality servo control of friction torque through the position control of release bearing in diaphragm spring clutch. It can also obtain actuator moving velocity without differentiating position signal with noise. Based on the elastic properties of diaphragm spring, wave spring and release finger, release bearing side load feature were analyzed, nonlinear actuator state space equations were derived in order to design feedforward compensation. Meanwhile, Lipschitz nonlinear observer theory was applied to observe actuator position, motor speed and release bearing force. Simulation results demonstrate that the control algorithm is feasible and has satisfactory observation robustness: In the presence of deviation between the worm gear efficiency and actual one, although there are steady errors in position and speed observation, the former error is negligible, and the latter is no more than 10 r/min, both of which are acceptable;Noise is visible in the observed values of position and force observations, but does not affect speed observation;When the driven plate is worn, although position and speed observations are still desirable, there is steady error in force observation, which is no more than 25 N.