Green roofs have been recognized as an effective way to increase urban green space and improve building micro-environment. This paper investigated the thermal and energy performance of an extensive green roof under free-floating and air-conditioned conditions in Shanghai, a coastal city with hot and humid summer. A field experiment was conducted on two full-scale rooms of a building from 30/07/2014 to 31/08/2014. One room was covered by a green roof while the other one was covered by a common roof. The results show that solar radiation has the strongest correlation with green roof's cooling effect, and the correlation between solar radiation and heat flux through green roof is negative in free-floating condition but positive under air-conditioned condition. Outdoor temperature is the most correlated factor of green roofs temperatures but the least correlated one for green roof's cooling effect. Energy balance analysis indicates that evapotranspiration and long wave radiation dissipate most of heat gain for green roof, and soil water content has a significant impact on energy balance. This paper presents the basic thermal characteristic of extensive green roof in Shanghai and also make it possible for further validation of hygrothermal transfer model of green roof system. (C) 2016 Elsevier B.V. All rights reserved.