Supplementary firing system is one of the common methods of increasing the power generated of combined cycle units. Low cost of investment to the rate of increase of the generative power has encouraged the designers to use this method in the above power plants. In this article, field study of the performance changes of a real combined cycle unit with and without supplementary firing has been performed from energy and exergy viewpoints. Studies show that in all operation modes subject to research, using supplementary firing causes an increase in power generation up to 26.3MW, energy efficiency of steam cycle about 2.43% and decreases the exergy destruction of steam flow control valves. But this system has a negative impact on energy and exergy efficiencies of the whole combined cycle, which at the worth case about 1.17% decreases energy efficiency. In addition, it was specified that the power plants operation in the partial loads causes high exergy destruction in the cycle which steam cycle increased energy and exergy efficiencies due to use   supplementary firing system cannot fully compensate it.