This study is a part of a refurbishment project, which renews the hydro-mechanical speed governor of a hydroelectric power plant (HEPP) with a new digital one. Based on the previous study of the project, which covers the validation of the HEPP model, the simulation studies implemented in Matlab/Simulink environment are conducted to design the best controller philosophy for the digital speed governor controller. In order to be able to give satisfactory response to the load frequency control of the power system, the speed governor should respond to the set point changes as quickly as possible within the safe margins. In order to compensate unsatisfactory performance indices of conventional proportional-integral (PI) controller, a new controller based on the differential flatness theory is designed. The new controller is constructed by adding on a flatness-based feed forward part to the existing PI feedback controller, which results in improved performance compared to the conventional PI controller.