This paper concerns the nonlinear observer-based control design, which is mainly based on Fliess(1) linearization and parametrized transformation for uncertain nonlinear processes. The proposed control scheme allows us to adjust the rate of tracking and the decay of observer error for regulating the system output. Through the valid selection of tuning parameters of the output feedback controller, the bounded output tracking is achieved and its tuning scheme is robust against model uncertainties. The effectiveness of our design method is demonstrated by a simulation example of the control of an exothermic CSTR with multiple steady states. Under significant observer-initialization errors and unstable operating conditions of designs, the satisfactory output regulation and disturbance rejection are obtained.