It is necessary to maintain or improve the thermal efficiency and decrease the maintenance cost in thermal power plants. For that, it is necessary to investigate the performance deterioration factors and solve the problem promptly. However, it is difficult to adequately understand the internal state of the plant and to find the performance deterioration factors only by the measurement data. Therefore we developed the method and the program based on heat and mass balance analysis to calculate the immeasurable quantity of state, efficiency of each equipment in a thermal power plant and the influence that these factors give to thermal efficiency. Moreover the performance of the plant can be simulated when the operating condition is changed. By using the method, we came to be able to investigate the performance deterioration factors in power plants.