The cost accounting of electricity and heat produced from an energy system is important in evaluating the economical efficiency and deciding the reasonable sale price. The OECOPC method, suggested by the author, was applied to a 650 MW combined cycle cogeneration system having 4 operating modes, and each unit cost of electricity and heat products was calculated. In case that a fuel cost is ${\}400/kg$ and there are no direct and indirect cost, they were calculated as follows; electricity cost of ${\}23,700/GJ$ at gas-turbine mode, electricity cost of ${\}15,890/GJ$ at combined cycle mode, electricity cost of ${\}14,146/GJ$ and heat cost of ${\}6,466/GJ$ at cogeneration mode, and electricity cost of ${\}14,387/GJ$ and heat cost of ${\}4,421/GJ$ at combined cycle cogeneration mode. Further, these unit costs are applied to account benefit on this system. Since the suggested OECOPC method can be applied to any energy system, it is expected to contribute to cost accounting of various energy systems.