This paper describes the dynamics and control of a spacecraft with flexible structures in gravitational field. We propose a simple model of the spacecraft. The motion of the model is controlled only by reaction wheels. Focusing especially on the time for damping, the effect of control gains for suppressing the vibrations is examined. The effect of three-axes attitude control of the model at various attitudes in gravitational field is clarified, considering the gravity gradient torques. Simulation results of orbital motion suggest two important conclusions. One is that the orbital periodic vibrations occur due to the reaction wheels at a certain attitude. The other is that the amplitude of the orbital periodic vibrations depends on the control gains for the attitude angles of the rigid body.