A digital turbine control system (TCS) has been developed for retrofitting an old analog TCS in a nuclear power plant. The developed TCS, which controls the speed of a turbine and the power load of a generator, is based on a triple modular redundant structure to ensure the system reliability. In addition, a turbine simulator has been developed to verify the perfection of the TCS prior to its actual installation. The simulator is composed of a graphic editor, a component model builder, and a system simulation solver. The tested TCS has been successfully applied to a CANDU type nuclear power plant. This paper describes major features of the developed TCS and the turbine simulator including thermal-hydraulic models. Also, the simulation result in a laboratory is compared with the pre startup simulation and the actual operation result.