The observer-based robust fault detection filter design and optimization for networked control systems(NCSs) with uncertain time-varying delays are addressed. The NCSs with uncertain time-varying delays are modeled as parameter-uncertain systems by the matrix theory. Based on the model, an observer-based residual generator is constructed and the sufficient condition for the existence of the desired fault detection filter is derived in terms of the linear matrix inequality. Furthermore, a time domain optimization approach is proposed to improve the performance of the fault detection system. To prevent the false alarms, a new threshold function is established, and the solution of the optimization problem is given by using the singular value decomposition(SVD)of the matrix. A numerical example is provided to illustrate the effectiveness of the proposed approach. The observer-based robust fault detection filter design and optimization for networked control systems (NCSs) with uncertain time-varying delays are addressed. The NCSs with uncertain time-varying delays are modeled as parameter-uncertain systems by the matrix theory. Based on the model, an observer-based residual generator is constructed and the sufficient condition for the existence of the desired fault detection filter is derived in terms of the linear matrix inequality. Furthermore, a time domain optimization approach is proposed to improve the performance of the fault detection To prevent the false alarms, a new threshold function is established, and the solution of the optimization problem is given by using the singular value decomposition (SVD) of the matrix. A numerical example is provided to illustrate the effectiveness of the proposed approach .