Abstract: The design problem of robust sliding mode observer for a class of nonlinear uncertain systems satisfying Lipschitz condition is considered, and a new nonlinear robust sliding mode observer design method is proposed by combining Thau observer with WalcottZak observer. By means of Lyapunov theory, it is shown that the estimation error will uniformly ultimately converge to a bounded set. Moreover, the designed observer is applied to the fault detection. Finally, the effectiveness of the proposed method is further demonstrated via numerical simulation.