In this study, the degree of performance changes between the guaranteed performance and the performance after a certain operating start time is calculated by using the performance test of gas turbine CHP. The reason of the performance degradation will then be analysed. For some results of the CHP plant performance tests the comprehensive electric power output was 8,380 kW lower than the guaranteed performance, and the gas turbine’s output was reduced to about 250 kW whenever ambient temperatures rose to 1℃. Also, causes of the performance degradation of gas turbines are ambient temperature rise, temperature aging and air compressor’s efficiency drop.