Abstract: Methods of error compensation for azimuth holding system in an INS (inertial navigation system) are presented. Two mutual orthogonal accelerometers are put in the azimuth holding system measuring the pitching angle and rolling angle. The attitude, angular velocity of the earth rotation and gravity are projected to the gyro coordinate system. According to the DTG (dynamically tuned gyro) that is drift sensitive by gravity, the azimuth holding system is compensated from bracket error, earth rotation and gyro drift associated to gravity. The results show that the drift of the azimuth holding system using the above methods is less than 0.06°/h.