In this thesis, three nonlinear excitation controllers have been designed to enhance transient stability of multi-machine power system. The first two types of controllers are based on full and partial feedback linearization of the system while keeping the nonlinearity origin of the system. This leads to a more accurate representation of the system than that in the case of conventional linear controllers. The third type of excitation controller is a newly designed controller called the fuzzy logic controller. The design of this controller is being based on fuzzy approximations of nonlinear functions of the system in hand. This is really helpful, since it allows the control engineers to design a nonlinear excitation control without the knowledge of the actual mathematical model of the multi-machine power systems. It is shown that the fuzzy controller design method ensures the stability of the nonlinear dynainics of the system. The design method is applicable to any multi-machine power system but tested on a 2 machine IEEE 14-bus test system. The performance of the proposed control scheme to large disturbances is evaluated, through computer simulation, and compared with the partial and full feedback linearization controllers.