This paper presents a robust nonlinear state feedback linearization approach for power systems with uncertainties. The uncertainties considered here are a class of bounded perturbations to the state model of power systems. The control law is applied through the excitation system of the synchronous generators connected to the power systems to enhance the stability of the system. In this paper, uncertainties are considered for single machine infinite bus (SMIB) system. A Lyapunov-based concept is used to guarantee uniform ultimate boundedness.