Abstarct: In this thesis, eigenstructure assignment method is used for sensor fault diagnosis of an unmanned aircraft. Since the aircraft open-loop system is unstable, first the state feedback is designed for the aircraft system using eigenstructure assignment. The proposed method is observer-baxsed and since the system is multiple-input multiple-output (MIMO), designing an observer for each output is needed. We also use eigenstructure assignment to design the observers and to obtain feedback gains. The simulation results reveal that the designed observer is very efficient in state estimation. After designing the observers, in the presence of faults, we use the error vector between the estimate of the state and the true state of the plant to reconstruct the faults. Then, fault reconstruction using the proposed method is compared to pole placement method which is a powerful benchmark. The Simulation results clearly show that the proposed method is perfectly capable of fault reconstruction.