Abstarct: As well as passing the time and progress in science, control systems become more complicated, security and reliability of these systems are taken into consideration. Due to problems and damages of operator fault in a system, this thesis tries to detect such a fault and its goal is estimation of system input states. Fault detection is done through sliding mode observer which its performance has been improved by fuzzy logic. Proposed method is a fuzzy system in which with consideration of uncertainties, it has more similarity to the main system. Finally, validation of proposed method is proved by Implementation of the method on a dynamic vehicle model, fault detection and estimation of its states.