Abstarct: Current modern societies depend on industrial systems intensively. With the growing of complexity and progress of systems and use of them in the sensitive environments such as flight control, powerhouses and chemical plants, increasing tendency has been created in field of fault detection. If the fault is not timely detected then significant part of equipment, information and in some cases manpower losses and damages. This thesis presents designing of fault detection and isolation system baxsed on observer. After glancing review upon fault detection, the structure of Luenburger observer and extended state observer would be introduced. Then use of fuzzy logic in designing of extended state observer is suggested for computation of the observer bandwidth. We use the composition of extended state observer and fuzzy logic in program of robust sensor fault detection. The results of simulations show effectiveness of usage of fuzzy logic in observer design.