In this paper, a robust sensor fault detection and isolation in load frequency control loop of interconnected power systems based on Unknown Input Observers (UIO) is designed and applied. The disturbances are assumed as unknown inputs to the UIO and the UIO is robust to disturbances. Simulations are performed for the dynamical model of a power control system composed of two areas. The proposed scheme is able to detect and isolate sensor faults. By using residuals, the designed "Fault Detection and Isolation Logic" system shows the operator which sensor is faulty. Hence the faulty sensor can be replaced by a healthy one for a more reliable operation.