In this article, a mathematical analysis of a CCHP system following electrical demand management and thermal demand management in comparison to separate heat and power system for an office building in Tehran is presented. In the thermal demand management, two modes including with selling electricity to grid and without selling electricity to grid are investigated. In order to have a comprehensive evaluation of the performance of the CCHP system, four criteria including primary energy saving, CO2 emission reduction, operational cost reduction and rate of return are employed for a typical office building in Iran. Also the AHP method is used to specify the best operation mode of the CCHP system when all of the criteria are involved. Results show that from the energy and environmental viewpoints, the CCHP system following thermal demand management with selling electricity to the grid is the best operation mode. But the CCHP system following electrical demand management has the maximum rate of return. For the all of the operation modes, the CCHP system has lower operational cost than SHP system throughout the year. From an integrated view point, the CCHP system following thermal demand management with selling electricity to the grid is the most attractive option.