In the single-axial rotation inertial navigation system using ring laser gyro (RLG),the errors of the inertial elements can be counteracted automatically because of rotating around the z-direction axis of inertial measurement unit (IMU),so the performance of single-axial rotating inertial navigation system is mostly determined by the estimate precision of z-direction gyro drift. A method to estimate the drift of z-direction gyro based on attitude determination is put forward in this paper,and the theoretical and experimental results are obtained. The experiments show that this method in this paper can estimate the drift of z-direction gyro efficaciously,and the result of navigation is excellent.