To prevent an electric power system losing synchronism under a large sudden fault and to achieve voltage regulation are major objectives in power system design. This paper applies the Riccati equation approach, together with the direct feedback linearization (DFL) technique, to design robust nonlinear controllers for transient stability enhancement and voltage regulation of power systems under a symmetrical three-phase short circuit fault. A DFL excitation controller and a DFL coordinated controller (excitation and fast valving controller) are proposed. The simulation results show that a power system can keep transiently stable, even when a large sudden fault occurs at the generator terminal. The robust nonlinear DFL controllers proposed here can greatly improve transient stability and achieve voltage regulation.