This paper presents a novel method to predict the outdoor ground surface temperature, which can be useful in the estimation of urban heat islands (UHI) eect. A model is developed based on a multi-layer ground surface energy balance calculated on an hourly timestep. It makes use of a shortwave and a longwave irradiation model, evapotranspiration model and an energy ux model in the soil. The latter is based on an electrical analogy using resistances and capacitances (thermal-RC model). The resulting ground surface temperature model has been implemented using an implicit numerical method in the CitySim software. Two dierent approaches (a detailed multi-layer and a lumped single layer method) have been analysed and their respective applications have been discussed. The ground surface temperature simulated for a case study in Switzerland was veried using eld measured data, and showed that in general the model reproduces well the surface temperature. The integration of the evapotranspiration model in the energy balance and the potential of green areas in mitigating UHI are analysed. To determine the sensitivity of the evapotranspiration model, the impact of dierent meteorological variables such as solar radiation, air temperature, relative humidity and wind speed on the evapotranspiration phenomena have also been investigated using the model.