This paper presents a shape transformation control method of a mobile robot with wheel-arms. The proposed method aims at transformation from a four-wheeled mode for high speed mobility to an inverted pendulum mode, which has advantages of high viewing position and small turning radius. The transformation starts with lifting up the wheel-arms to raise the center of gravity of the whole body. From such initial states, the body is lifted up and controlled to the target angle by partial linearization, while returning the arms to the initial angle. Then, the wheels are slowed down by properly changing the target pitch angle of the body. Stability analysis is carried out by taking into account the transient response of the body angle to the target value, which is neglected in existing work. The effectiveness of the proposed method is demonstrated by experiments.