As a part of recent CO2 emission reduction studies in the concrete industry with active use of concrete admixtures with low basic unit of CO2 emission such as blast furnace slag (BFS), basic unit of CO2 emission by SBFS was computed in order to assess CO2 emission by reinforced concrete building with smart blast furnace slag (SBFS). In addition, SBFS concrete was applied to the subject building for assessment of CO2 emission during material production step among construction steps. Life cycle CO2 emission assessment on the subject building was classified into 7cases according to mix ratio of BFS and SBFS.