The conventional proportional-integral (P-I) controller for the steam generator water level has no feedforward action at low powers because of the large flow errors, which cannot effectively reduce the swell and shrink phenomena. The steam generator water level is the sum of three water level quantities that are induced by the mass capacity effect, the swell and shrink phenomena, and the mechanical oscillations. Each individual quantity of the three water levels is not measurable. The water level quantity due to the mass capacity (water inventory) is important for cooling of the primary side, which is an essential function of steam generators. Therefore, the flow errors and the unmeasurable water level quantity are estimated and then used to generate the control input (feedwater flow rate). Under practical situations, the proposed controller is predicted to have a better performance than the conventional P-I controller.