Due to a variety of effects, such as lightning, severe storms and equipment failures, the configuration of an electric power system is not constant. Thus, important parameters, such as effective reactances of transmission lines, in an electric power system always contain some uncertainties. The aim of this paper is to discuss transient stability enhancement and voltage regulation of power systems using feedback linearization techniques and an adaptive control approach. A nonlinear controller design approach is employed to design an adaptive nonlinear control law to prevent the loss of synchronism in a power system under a large sudden fault. An adaptive nonlinear controller is proposed and the stability results for the overall system are established. Simulation results show that transient stability enhancement and voltage regulation of a power system under a large sudden fault can be achieved.