In this paper, spacecraft kinematics and dynamics for a rigid body in circular orbit were studied to recognize the system how operates in geosynchronous orbit for linearized attitude motion. The linear model of flexible spacecraft was figured out with linearized rigid spacecraft attitude dynamics, linearized gravity gradient torque and a sinusoidal effect as a disturbance which consists of flexible solar panels vibration effects, sun pressure and other unmodelled external or internal disturbances. Satellite's linear attitude motion is presented in the state-space form. State-space linear controller is designed by using pole placement method. Designed system is simulated by using Matlab-Simulink. Simulation results show that a satellite unstable attitude motion is successfully stabilized (with settling time about twenty minutes) by designed linear controller. Other control performances such as overshoot, steady-state error, etc., are also very good.