Because of the energy crisis and the low proportion of new buildings in Taiwan, rational use of electricity in existing buildings becomes very important for reducing carbon emissions and combating global climate change. The retrofitting of lighting systems is one of the most widely adopted energy saving measures. This study presents the potential energy saving and payback period of lighting system retrofit through replacing low performance T8 fluorescents with T5 fluorescents and LED indicator lights in the Green Building Renovation Plan. Based on the analysis, this study concludes that lighting system retrofit can achieve significant energy saving ratios of 10-17.5% in daily use. However, the average payback periods of GBRP investment and normal purchase investment are ten years and 7.82 years, respectively. Both exceed the average lifetime of lighting equipment, suggesting that lighting system retrofit is slightly uneconomical because of the low electricity tariff in Taiwan.