|The importance of the evaluation and verification of small-size cogeneration system has been emphasized because there is no KS-code related to the small-size cogeneration system. The evaluation method of small-size engine cogeneration system, regarding Japanese standard JIS B-8122 and international standard organization, ISO-8528, was applied to the system. The evaluation methods, start-test, rapid-load-up and rapid-load-down, etc. were executed at the system, and reasonable results were acquired. The electrical and thermal efficiencies were executed and analyzed at various load conditions. The NOx emission at various load condition was also measured. Finally, the gas engine cogeneration system was installed to a site for actual usage and it was continually operated during more than 6 months as the site condition.