The actuator and component fault is always regarded as a whole for the fault detection and diagnosis in control systems, which does not favor the decision and processing. The sliding mode observer is designed for the nonlinear fault diagnosis and the method is proposed for the fault diagnosis of the component. The whole nonlinear system and the actuator system are simultaneously observed. The faults of the sensor, the actuator and the component are diagnosed according to the residual value changes of the two observers. Finally, the single-input single-output nonlinear system is simulated, which verifies the effectiveness of the proposed method.