This study aims to analysis domestic energy consumption in Korea and reduction greenhouse gas by building mechanical system. At this point be tormented the energy depletion and climate change of earth are big problems on the eatrh. In this paper we will find out best methods to reduction greenhouse gas and energy consumption by practical building mechanical system. Enlargement of greenhome and building adopt, greenhouse gas exhaust reduction in building, publication of energy consumption rate, publish building energy management manual, etc.