This paper proposes an actuator fault diagnosis approach based on unknown input observers for the attitude control system of satellite. More specifically, by considering an actuator fault as the unknown input vector, an unknown input observer is designed so that the residual is sensitive to the other faults while insensitive to the appointed one. In this work, the design of unknown input observer is formulated as a set of linear matrix inequality (LMI), which can be solved by the LMI toolbox conveniently. Finally, the proposed approach is applied to the satellite attitude control system. Simulation examples illustrate the presented method is able to detect and isolate the actuator fault effectively.