Reduce energy consumption and environmental pollution is an important objective in energy sustainability. The building sector, one of the fastest growing in terms of energy consumption, accounts for over 40% of final energy. Building energy conservation will drive the application of new energy conservation technologies and strongly promote the development of sustainable building. In this paper, the extenics theory and life cycle assessment (LCA) are proposed in building energy conservation assessment. Analytic hierarchical process (AHP) method and 9-scale pair-wise comparison are adopted to determine the weights of the factors in different hierarchies. A building energy conservation assessment model combining LCA and the extenics theory is established. The matter-element and the dependent function are defined. Then the synthesis dependent degree and the final grade index are calculated. Thus the building energy conservation grade is obtained. An example is used to illustrate the proposed approach. The results provide guidance to assess building energy conservation performance and determine the energy conservation grade of buildings. (C) 2009 Elsevier Ltd. All rights reserved.