Nowadays, Because of the sharp decline in fossil resources, producing power by renewable energy is growing rapidly. Photovoltaic (PV) technology is one of the most popular ways for generating electricity. In hot days of year, which the maximum irradiation of sun is available, because of high cell temperature, the efficiency of PV cells is falls down. In this paper, in order to decrease the temperature of PV cells, the use of carbon nano fluid for cooling the PV panel is investigated. The results show that after 4. 5 hours of beginning the test, the temperature of PV panel with carbon nano fluid cooling is fixed at 32° C, while the temperature of conventional panel is about 83° C. This temperature difference improved average efficiency around 5. 75%. Moreover, in this study, calculation of output power of a 10-KW PV power plant, with and without cooling is performed.