The angular rate signal output from gyro is proportional to that input from vehicle, which directly affects the accuracy of the navigation system. As the Fiber Optical Gyroscope developed at home and abroad is improving in accuracy, the application of FOG in strap-down inertial navigation system(SINS)has become popular. In many applications, but, the scale factor relative to the FOG accuracy is not completely linear with positive and negative asymmetry, which obviously have an impact on the accuracy of navigation system. To further improve the accuracy, an approach to precisely calculate the FOG scale factor, its nonlinearity and asymmetry is presented, by which a rate experiment is finished. The programmed software is used to calculate such parameters as the FOG scale factor