Abstract: The networked control system (NCS) deals with how to realize close-loop control through network. It is a multi-subject research region involving control science, computer net- work, etc. In this paper, the networked control system is regarded as a sampled-data control sys- tem with time-delay. A memoryless reduced-order state observer is constructed. Then the residual is generated by comparing the output of the observer with the output of the practical system, and faults of the NCS is thus detected. Simulation result shows the approach is feasible.