In the electro- hydraulic servo system,extra sensors are often added for fault detection and isolation (FDI),but the sensor fault can cause a false alarm.The electro- hydraulic servo system itself is a typical nonlinear system and is often subjected to time- varying and unknown disturbances,which brings about great challenges to the sensor FDI.In order to solve these problems,a FDI scheme based on a nonlinear robust observer is proposed.In this scheme,the robust observer is used to handle the system nonlinearity as well as unknown disturbances and a linear matrix inequality method is adopted to facilitate the observer design.For the sensor fault isolation,a batch of robust observers is designed and some logic rules are made.Then,the proposed FDI scheme is verified by simula- tions and experiments,and an adaptive threshold is designed to make decision according to the characteristics of ex- perimental data.Both simulation and experimental results show that the proposed FDI scheme is effective.