In networked control systems the information is exchanged using a network among control system components. The performance of the closed - loop system is not only determined by the characteristics of the control system, but also determined by the scheduling manner imposed by network. The designer of fault diagnosis should take the new peculiarities into account. In this paper, networked control systems are modeled based on information scheduling scheme and the fault detection problems of them are studied. The residual signal is generated using an unknown input observer. Sufficient conditions for the existence of unknown input observer are derived. Several stability results regarding the state estimation error are established. Simulation results are given to show the effectiveness of the design.