A nonlinear passivity based decentralized excitation controller for multimachine power system is designed to stabilize the speed and regulate the terminal generator voltages. To design a control sliding function the passivity technique is used. Then, with this function a sliding mode integral controller is proposed. This combination enables to reject internal and/or external perturbations and take advantage of the energy properties of the power systems. The designed controller was tested through simulation on an equivalent of WSCC (Western System Coordinating Council) under small and large perturbations. The presented simulations confirm effectiveness of the proposed controller.