Abstract: A class of networked control systems with uncertain long time delays of asynchronous sampling clock is considered,and a discrete state observer is designed to detect faults when they occur in the system.When the system is normal,the stability condition of the observer system is approximated with a minimization problem involving conditions of linear matrix inequalities.So long as this minimization problem is solved,the whole system is asymptotically stable.When a fault occurs,the observer residue can change rapidly to detect the fault.A simulation example is given to demonstrate the effectiveness of the proposed method.