摘要：This paper proposes the financial index of payback period analysis in order to evaluate the economic performance of lighting system. Conventional designs and proposed optimal design using the Energy Conversation Promotion Act （No. 2） B.E. 2550 of Ministry of Energy （Section--The Type or Size of Building Standards and Rules and Procedures, Building Design for Energy Conservation, Act B.E. 2552） are compared based on illumination standard for office building in term of energy consumption, return of investment and environmental impact. Firstly, the Type or Size of Building Standards and Rule and Procedures, Building Design for Energy Conservation, Act B.E. 2552 is reviewed and compared with the conventional designs of lighting system for office building. Next, the lifetime energy consumption of fluorescent lamp T5 with electronic ballast, fluorescent lamp T8 with standard ballast and fluorescent lamp T8 with electronic ballast are simulated using DIALux software program in three sizes office buildings. Then, the payback period and CO2 emission of lighting system for selected office building are simulated by various scenarios, which the analytical results show are the optimal lighting system for office building under the present situation. Finally, the optimal design of lighting system for designated office building in Thailand will be discussed with respect to the energy performance with environmental friendly.