Recently, there are growing interests in building life cycle assessment in response to climate change which is emerging as important issue. However existed assessment method is insufficient for satisfying characteristics of buildings. Thus it is required that effective assessment system which is suitable for reducing $CO_2$ emissions. Therefore, the purpose of this study is to develop building life cycle $CO_2$ assessment system according with required elements and building life cycle characteristics. To that end, the strategies of the system development was established for developing element technologies which are basis of the assessment system. And core values which are consist of keyword limited the direction of the development and the boundary of the technology. On such basis, element technologies were developed. Web-based assessment technology was based on the cycle of calculation and analysis and data accumulation by web system that each assessor is connected to online server. And it can be utilized as a building $CO_2$ assessment hub which combines the BIM modeling and the result of the other analysis tools. Automated materials quantity load technology was based on $CO_2$ functional unit and the list of building materials quantity which contains work classification and building material names and costs. And it can be calculated by data mapping technology through standard database format. Alternative assessment technology was based on life cycle template system. Also each element technologies are combined for building life cycle $CO_2$ assessment technology. As a result, Building life cycle $CO_2$ assessment system that includes effective calculation algorithm and alternative assessment technology and analysis system can be developed.