© Copyright 2017 Inderscience Enterprises Ltd. A robust load frequency controller is applied to a nonlinear three-area interconnected power system where non-reheat, reheat, and hydraulic turbines are distributed in different areas respectively. The nonlinearities such as governor dead band and generation rate constraint are included in the block diagram model of the power plant. A sliding mode controller (SMC) is developed to regulate frequency error and tie-line power error of the power system despite the presences of load changes and parameter variations. The SMC is simulated on the nonlinear and three-area interconnected power system. The simulation results demonstrate the robustness of the control system against external disturbance and system uncertainties. They also show that the SMC successfully drives the frequency error and tie-line power error to zero.