A sliding mode decentralized excitation controller for multimachine power systems is designed to stabilize the power angle and regulate the terminal generators voltages. First, the block control approach is used to derive a nonlinear sliding manifold, on which the mechanical dynamics are linearized. Then, a combination of the integral and nested sliding mode techniques is applied to reject both the matched and unmatched perturbations. The designed controller was tested through simulation of WSCC (Western System Coordinating Council) under small and large perturbations. The presented simulations confirm effectiveness of the proposed controller.