The reinforced concrete (RC) buildings commonly used in Taiwan not only create great pollutions in material manufacturing and construction phases but also destroy the environment. On the other hand, the light weight steel buildings are safe, healthy, comfortable, producing less waste, and environmental friendly. Therefore, light weight steel buildings have been promoted in Taiwan by the government as an important “green building” policy. In Taiwan, there is still a large market of low rise light weight steel housing. To promote light weight steel housing in Taiwan, we should evaluate its influence on environment. In this research, we established a CO2 emission database for light weight steel building materials and calculated CO2 emission for a light weight steel house. The results showed that a low rise light weight steel house has 39% less CO2 emission than an RC house in the same scale. A light weight steel house has a good building envelope that decreases energy consumption of air-condition by 35.42-42.95%. Therefore, a light weight steel house has less CO2 emission from building materials and energy consumption than an RC house.