Abstract: The strapdown optic fiber gyro inertial navigation system is studied. A method to rotate the inertial measure unit is presented to improve the system accuracy. Error features of the gyro compass is analyzed. For the error of the rotation unit, a finite memory Kalman filter is applied to estimate the states during the initial alignment. It improves the accuracy of the alignment. Simulation results show that the mean square error of initial yaw alignment is about 0.159°.