The growing threat of global warming and energy shortage shave created an urgent demand for reconstructing outdated building structures; the "green roof", to this effect, has garnered a great deal of research attention. In this study, we evaluated the financial benefit of fitting the Computer Science & Information Engineering (CSIE) Building at National Taiwan University with a green roof. By modeling the building's energy consumption in eQUEST software, we established an energy-saving reconstruction scheme and found that the green roof is most cost-effective when the area over the air-conditioning system was paved; further, among the several factors included in the model, we found that increase and decrease of electricity usage was the factor with the most influence on the evaluation index.