Micro grid energy system is consisted of various kind of electric generator (ex: wind power system that use natural energy, gas engine, storage battery and etc.). These devices are needed to be operated in line for the electricity demand change. Fuel composition also changes in assuming biomass use in micro grid. So, it is important to understand their transient response properties for designing and managing micro grid energy system. In this study, gas engine dynamic model was built to predict transient response and it was also investigated in experiment for existent gas engine. In addition, influence of combustion duration, which was affected by fuel property, on transient response was investigated by using engine dynamic model.