The aim of this study is to investigate the energy performance of a medium scale green roof system installed on a two-story commercial building in Athens, Greece and its possible contribution to the mitigation of the urban heat island phenomenon. For the purpose of this research, in-situ measurements of temperature and humidity took place during the cold period of the year. The surface temperature of the conventional concrete floor of the roof was found to be up to 21.9 K higher than the one of the planted area during the day, while at the first hours of the day, the surface temperature of the green roof appeared to be up to 1.6 K higher than the one of the cement floor of the roof. Differences of the surface temperatures of the varying plants of the green roof were measured and led to a proposal of an optimum selection among them for the best performance of such a green roof system. Furthermore, simulations for this building were performed by using the EnergyPlus program. It was calculated that, the indoor air temperature, for a non air conditioned status of the building, was decreased up to 1.1 K during a typical summer day and was increased up to 0.7 K during a typical winter day. Lastly, an overall saving of 15.1% for a whole year on the energy consumption of the building was calculated. (C) 2016 Elsevier B.V. All rights reserved.