The main objective of this paper is to describe the design of proportional integral observer (PIO) for Takagi-Sugeno system using the nonlinear sector approach. This observer improves the state estimation due to the integral part. Lyapunov functions are used to analize the stability of the error dynamic of the observer. Stability conditions are obtained in terms of linear matrix inequalities (LMIs). The effectives of the proposed approach is illustrated by knee rehabilitator robot.