The thermal and nuclear power plants have been imposed a stable supply of electricity To certainly achieve this, we built the plant condition monitoring system based on the heat and mass balance calculation If there are some performance changes on the turbine system components of their power plants, the heat and mass balance of the turbine system will change This system has ability to detect the abnormal signs of their components by finding the changes of the heat and mass balance Moreover we note that this system is built for steam turbine cycle operating with saturated steam conditions