Abstract: A fault diagnostic method for actuators in nonlinear systems based on T-S fuzzy model and parity equation is discussed in this paper. In order to detect and isolate actuator faults, fully-decoupled parity equations are designed to generate residuals. The residual which is sensitive to a special actuator fault is decoupled from system states, disturbance inputs and other actuator inputs. The existence condition of fully-decouple parity vectors is presented. T-S fuzzy model is used to describe nonlinear systems and the fuzzy parity equation of nonlinear system is given. The faults in actuators are represented as biases and scale factor changes. A parameter estimator based on Kalman filter is designed to identify biases and scale factor changes from the information contained in the residuals generated by fuzzy parity equations. A simulation example of a nonlinear aircraft model is given for illustration.