In the present study, the WRF model with the incorporation of a Single Layer Urban Canopy Model is used to investigate the cooling effect of roof greening in Nanjing. Sensitivity tests of albedo, heat capacity, thermal conductivity and humidity are conducted for the time period from 28 to 29 July 2013. The simulation results show that: (1) the albedo of roofs is reduced to approximately 0.15 after greening, which is smaller than the albedo values of the roofs built with cement or other reflective materials. This result may lead to the increase of city temperature by 0.2℃ in daytime. (2) The heat capacity of roofs increases significantly after greening, leading to the decline of temperature by 0.33℃ in daytime while the temperature at night increases by about 0.21℃. (3) Due to the existence of vegetation and soil layer, the thermal conductivity of roofs is weakened. As a result, the net radiation on roofs is more easily converted to sensible heat rather than being transferred downwards, warming the overlying ambient air. (4) The change of soil humidity enhances the release of latent heat from the net radiation on roofs. Therefore, temperatures in daytime and nighttime are reduced by about 1.23℃ and 0.44℃, respectively. In addition, the statistical analysis of the simulation results for different seasons shows that the reduction of temperature by roof greening is the strongest in summer, which can reach 1.22℃. In comparison, the temperature decreases by about 0.96℃ in spring, 0.75℃ in autumn and only 0.38℃ in winter.