Abstract: The relative internal efficiency for steam turbine is one of the main economic indexes in evaluating the flow path conditions of steam turbines.The steam turbine used in nuclear power plant works in wet steam area.The relative internal efficiency may not be obtained accurately through measuring of the wetness of regenerative extraction steam and exhaust steam.In this paper,by analyzing and comparing the enthalpy dropping and power types relative internal efficiency,it is found that when the operation conditions of the flow path in steam turbines change,both the two kinds of relative internal efficiency can reflect the operation conditions of the flow path in steam turbines.Also,when the operation conditions of regenerative system change,the enthalpy dropping and power types relative internal efficiency are equivalent.So either one can be used as the economic index for nuclear steam turbine.