Abstract: In this paper the power system load frequency Control is examined from the viewpoint of stochastical control theory. Identification method is first used to build models of power demand disturbances. Next, hierarchical estimators are constructed to estimate and predict the disturbances.Finally the load frequency controller is derived on the basis of invariance principle. Simulation of a simple system consisting of only one steam unit and one hydro unit has demonstrated much better performance of the proposed controller than the conventional integral one under various types ofdisturbance conditions.