The paper analyzes CO_2 reduction effect and economic effect of cogeneration network system (CGNS) with considering grid limitation in electric quality. In CGNS, cogenerations are connected to the grid, being allowed to send excessive electricity to the grid. The system equips heat storage system and it is operated to contribute to reduce power-demand fluctuation; in this sense the system is cooperative to the grid. Analysis was made to compare the system with conventional cogeneration system without networking. A model area based on the actual service area in a city was selected and analyzed. The result indicates that the CO_2 reduction rate increases to 20% with the CGNS from 10% of the conventional cogeneration system. Additionally the power fluctuation in the grid can be also expected.