Abstarct: In this study an observer-baxsed adaptive fuzzy controller for drug dosage in cancer treatment is presented. In the controller design, the universal approximation property of the fuzzy system is used to approximate the unknown functions in the system model and simultaneous, a state observer is used to estimate the state variables. The stability of the closed-loop system is demonstrated using Lyapunov theory. The simulation results show the proper performance of the controller and state observer taking into account the unknown system dynamics. In the proposed method, in addition to controlling tumor cells, the effective immune cells are also at the desired level.