Two-wheeled mobile manipulator systems have many advantages compared to the statically stable systems. Due to the highly nonlinear underactuated structure (more degrees of freedom than the number of actuators), it is difficult to control the motion of wheels and stability of underactuated joint at the same time by wheels. In this paper, we deal with motion control of the wheels through CoG (Center of Gravity) manipulation of manipulator while keeping the stability of passive joint with backstepping method. In order to move wheels to desired position, inverted pendulum model is utilized to control the center of gravity position of manipulator. The trajectory of CoG is obtained by using the optimal linear quadratic method, which provides smooth CoG pattern motion. The preview control structure is utilized to improve the transient response and compensate the delay between input reference and output of wheels. The validity of proposed method is verified by simulation results.