Abstract: Optical sensors and inertial sensors are two kinds of important attitude measuring components.How to isolate the faults of these two kinds of sensors is a difficult problem in the fault diagnosis of the closed-loop attitude control system of satellite.There is analytical redundance between the outputs of these two kinds of sensors deduced from the kinematical equation,so a virtual system can be established.Two different observers can be used for this system,one of which is the Kalman filter used for detecting the fault of both sensors,the other is the isolating observer detecting the fault of optical sensors.Fault isolation can then be achieved by comparing the residuals of these two observers.The proposed method is applied to the satellite control system with sun sensors,infrared earth sensors and gyroscopes,and the mathematical simulation results validated the efficiency of the method.