A sliding mode based load frequency controller is applied to a two-area power system. Non-reheat and reheat thermal turbines are distributed in these two areas respectively. The nonlinearities such as governor dead band and generation rate constraint are included in the block diagram of a power plant model. Our control goal is to regulate the load frequency error of the power system in the presences of different load changes and parameter variations. The sliding mode controller (SMC) is simulated on the two-area interconnected power system with nonlinearities. The simulation results demonstrate the robustness of the control system against external disturbance and system uncertainties. They also show the convergences of load frequency error, tie-line power error, and area control error to zero with the SMC. In addition, the performance of SMC is compared with that of a conventional PI controller. The comparison study shows the superiority of the SMC to the PI controller in term of control performance.