Increasing demand of energy and adverse environmental impact of the use of fossil fuels emphasized the need of using renewable energy. At this stage, cogeneration has come into question. Cogeneration, also called combined heat and power, is a consecutive generation of electrical power and heat energy. It is an efficient and cost-effective means to save energy and reduce emissions of human origin. In this study, a combined cogeneration system has been investigated. This cogeneration system uses a combined gas/steam turbine and draws gap steam from the steam turbine for heat production. Thermodynamic analysis of the system has been done and effects of some design parameters on performance values have been examined.