The function of Automatic Generation Control (AGC) is to regulate the output power of the corresponding generator in response to changes in system frequency or tie line loading or in both cases within a prescribed limit. In this paper Differential Evolution (DE) based Proportional-Integral (PI) controller is designed and simulated to observe its performance for a two area Hydro-Thermal power system. The purpose of the DE is to find out the optimal parameter values of the PI controller (K p and K i ). The Optimal set of values is chosen based on eigenvalue of system matrix and objective function. The performance was evaluated based on the transient response (Settling time and peak overshoot) of the system while different step load changes were applied on both or either of the areas. The proposed controller has been found functioning properly for not only smaller (1%) but also larger (5%) load disturbances. All the simulations are done using MATLAB/SIMULINK software.