This study purports to construct a system that assesses the life cycle $CO_2$ emissions of a building according to the progress of the construction project and to thus develop a object-oriented building life cycle $CO_2$ assessment program that can execute a comprehensive evaluation of the $CO_2$ emissions resulting from construction activities. To achieve this, this study first investigated the standards of the ISO 14040 Series and trends in domestic and international studies on the life cycle $CO_2$ emission assessment program for a building after analyzing the study trends and necessary elements; then, items and methods of building life cycle assessment that are suitable to the conditions in Korea were chosen. The composition of the building life cycle $CO_2$ assessment program was subdivided into 4 systems : a simplified assessment system, a detailed assessment system, a site assessment system, and an assessment analysis system, according to the object of the building life cycle $CO_2$ assessment and by reviewing the range and timing of the building life cycle assessment by construction project phase. A database that can be applied efficiently to this study was constructed following an analysis of the computation technique for the $CO_2$ basic unit of each construction material and energy source as well as the characteristics of the construction materials depending on the usage, structure, and form of the building. As a result, the computation model and assessment technique for elements to be assessed for the simplified assessment system, the detailed system, the site assessment system, and the building life cycle $CO_2$ assessment system by construction project phase were obtained, and the object-oriented building life cycle $CO_2$ assessment program (LOCAS) was developed by integrating these systems.