The initial alignment is vital to the strapdown inertial navigation system (SINS). In this work, an efficient initial alignment method for SINS based on a six-degree of freedom inertial measurements unit (IMU) is proposed. The three dimensional linear accelerations and angular rates are acquired from a data acquisition board which contains a low-cost IMU. Preprocess the gyro data and modeling the SINS error model for initial alignment. Analysis the observability of this SINS error model and then simplify the model based on the observability. A kalman filter is conducted to estimate the misalignment angle. The results show that the initial alignment method proposed in this paper is advisable for a SINS based on low-cost IMU on a stationary base.