Abstract: The low precision characters of the integrated inertial/stellar attitude determination system consists of MEMS (micro electromechanical systems) gyros and CMOS APS star sensor. For this question, the methods of attitude determination from vector observations which suitable for this system were studied. There are several attitude determination methods suitable for the gyro/star-sensor configuration such as EKF, QUEST and optimal REQEUST. According to the different characters between the EKF and optimal REQUEST and together with the single-frame method QUEST, the quaternion was used as the attitude parameter. A two-segment attitude estimator was presented: when the estimated error of gyro drift was large, the fusion mode of EKF and QUEST was used to estimate the gyro drift as soon as possible. When the estimated error of gyro drift reduces to some low extent, the filter was switched to the fusion mode of EKF and optimal REQUEST. The results of simulation show that the two-segment estimator can estimate not only the attitude parameter but also the gyro drift. The determination accuracy of the attitude estimator can meet the attitude determination requirements of the most spacecrafts.