CO2 emissions associated with energy consumption in buildings contribute about 30% of the total CO2 emissions worldwide. The building sector is among the key sectors for the changes towards a low-carbon economy, and is also where the greatest economic potential of GHG emission reduction exists. Unique characteristics of the building sector, including complex stakeholder network, fragmented value chain, multi-phase life-cycle and slow turn-over, became barriers to the market transfer towards sustainable practice and effective emission reduction in this sector. Policies have a critical role to play for getting over these barriers. The proposed project is a comparative study of the stakeholder networks (including at least institutional structure, law-makers, government agencies, NGOs, developers, architects, engineers, construction companies, material manufacturer, owners and users), and the policy framework and policy instruments in the building sector in Japan and the U.S.. Global dialogue on climate change mitigation in the post-Kyoto era recognized sectoral approaches as an important instrument in addition to the economy-wide emission reduction commitment. This study will also explore how national/local policy instruments can be aligned with international/regional sectoral targets in the building sector. The two countries for comparison were purposefully chosen for their significant influence on the policy orientation and building practice in emerging economies, especially China and India. As the most energy-efficient nation, Japan provides examples for its Asian neighbors; whereas the US as the largest economy in the world is having tremendous impact on the ways countries like China and India choose to develop. Effective policy framework and instruments are urgently in need in these countries for curbing GHG emission and are crucial to the achievement of the low-carbon goal globally in the building sector. A synthesis of case study and comparative analysis methodologies is adopted in this study to investigate three main themes of research questions (status, impact and actions). Based on a comprehensive literature review and an archival study of the context, framework, detailed instruments and effectiveness of the emission reduction related policies in the building sectors in Japan and the U.S., specific urban and building projects with similar scale and magnitude of impact in the two countries will be selected for an in-depth exploration of the dynamic stakeholder network, the enabling and constraining policies for high performance, as well as the interaction between stakeholders and policy implementation, through archival study and structured interviews. The goal of this research is to understand the building emission reduction related policies in Japan and the U.S., mechanism behind different approaches, stakeholders’ impact and the resulted effectiveness of policies, as well as to explore characteristics of an effective policy framework in the building sector for climate change mitigation. Findings will contribute to the identification of barriers and challenges in the building sector of Japan and the U.S. for achieving the low-carbon goal, and provide constructive references and lessons for other nations, especially emerging economies like China and India, whose choices of development path will have a huge weight on the success of climate change mitigation of the entire world.