In recent decades, greenhouse gas (GHG) emissions, particularly CO2, has led to catastrophic environmental impacts such as global warming. In addition to natural emissions, human activities are among the most important causes of CO2 emission. Meanwhile, energy consumption possesses the biggest share in CO2 emission. Various sectors, including industry, transportation and building, consume energy which each one has a significant role in air pollution and GHG emissions. According to official statistics, buildings own more than one third of the world energy consumption. Therefore, the role of huge buildings, as big energy systems would be very important. In this paper, a model for CO2 emission caused by energy consumption in a large building is provided and the emitted CO2 from a real building as a case study is then calculated.