An approach to parameter optimization of PI sliding mode load control used for Automatic Generating Control (AGC) of multiarea power systems with nonlinear elements has been proposed. The method has the advantages of PI and sliding mode control. Instead of using traditional analysis algorithm to obtain the controller parameters, a single-input single-output (SISO) nonlinear optimization technology is introduced and a robust PI parameter formulae is derived. MATLAB Nonlinear Control Design (NCD) toolbox is used as the PI parameter optimization tool to deal with the interconnection of the SISO AGC loops. The nonlinear objects such as generation rate constraint (GRC) and deadband of the turbine governor are treated easily with the help of NCD. The proposed optimizing method is suitable for power system engineering applications. The simulation of a two-area power systems is reported and the results are reasonable.