The purpose of this paper is to simulate the performance of thin green roofs during summer in Seoul, where there are four seasons. Many experimental studies are available for the green roofs, but there have not been many analytical studies. Numerical analysis is applied to a simple model developed for this study. The effects of the parameters affecting green roof performance are studied using several roof types. The weather data from Korea Meteorological Administration for the summer in Seoul have been used as an input for the simulation model. The results agree with the trends reported in previous studies. The importance of insulation status of the roof before applying green roof and the climate conditions is confirmed. Analysis such as this paper is necessary and useful before applying green roof or planning experiment on green roof.