Green roof systems have become an important part in creating sustainable cities. They can provide a wide range of economic, environmental, and social benefits. The goal of this research was to quantify the thermal performance improvements from a green roof with mineral wool substrate installed on a school building in a humid subtropical climate. In-situ measurements during a summer period included heat fluxes through the green and reference roof, vertical temperature profile through both roofs, the local air temperature above roofs, and local meteorological parameters. Furthermore, the summer thermal performance of green and reference roof and the green roof cooling effect were evaluated concerning meteorological parameters using the Pearson correlation analysis. The results indicate that the green roof layers have improved thermal performance of the roof with respect to reduced conductive heat flow by 57% and delayed heat transfer. The maximum and averaged reference roof to green roof outdoor surface temperature difference was 27.5 degrees C and 5.5 degrees C, respectively. It was found that ambient temperature and relative humidity have a dominant role on the thermal performance of the green and reference roof, while solar radiation and ambient temperature present the key meteorological determinants of the green roof cooling effect.