The goal of this paper is to develop a nonlinear observer- based control strategy for a multi-variables continuous stirred tank reac- tor (CSTR). A new robust nonlinear observer is constructed to estimate the whole process state variables. The observer is coupled with a non- linear controller, designed based on the input-output linearization for controlling the concentration and reactor temperature. The closed loop system is shown to be globally asymptotically stable based on Lyapunov arguments. Finally, computer simulations are developed for showing the performance of the proposed controller. Continuous stirred tank reactors (CSTR) are ones of the most important plants in chemical industry. In practice, however, the control of these reac- tors poses a number of challenging problems. These problems arise from the presence of severe nonlinearities of these chemical reactors, as well as for mon- itoring and control applications, only a few measurements are available, either because the measuring devices do not exist or are too expensive. Therefore, we can deduce that the main difficulties arising in the control of CSTR's reactors arrive from two main sources: the process complexity and the difficulty to have reliable measurements of state variables ((1), (16)). In recent years, a various design methods of nonlinear control strategies have been proposed. Most of these based on differential geometric concepts ((15)-(18), (23)). This method allows a certain class of systems to be linearized using state feedback and co- ordinate transformations ((12), (14)). Extensions of the method like adaptive linearization ((21), (25)), robust linearization (24), and asymptotically exact linearization (13) account for small model-plant-mismatch.