Two wheels driven wheelchair doesn't have any front casters hence it is superior to normal wheelchair in terms of mobility. However, it cannot keep the stable attitude without control. Also, it is necessary to realize power assist control of yaw direction motion in order to obtain operability equal to normal wheelchair. In this paper, pitch angle disturbance observer (PADO) is utilized to realize robust stabilization. PADO can estimate disturbance of pitch angle. Estimated disturbance is compensated in Lyapunov-based controller which stabilizes the pitch angle. To realize power-assist control of yaw direction, reaction torque observer (RTOB) is applied to yaw direction. The validity of proposed method is verified by experiment.