Urban heat islands, temperature increase due to climate change and energy consumption due to high summer cooling load are significant issues in Turkey. International studies indicate that the green roof system serves as an energy efficient building technology. However, the thermal performance of green roofs when exposed to local climate conditions is still unknown in Turkey. A research project is being conducted at Istanbul Technical University, in which part of a low-slope existing roof system was retrofitted as an extensive green roof system and the thermal performances of both the existing roof and green roof were monitored in order to make a comparative assessment. Both the green roof and the existing roof were instrumented to measure the temperature profile within the roof systems and the solar reflectance of the roof surfaces. Local meteorological variables were also measured. Results obtained from the field monitoring revealed the following data. Reflected solar radiation from the green roof surface was slightly higher than from the existing roof surface. This was likely to be due to the fact that the plants had not yet covered the entire soil surface area of the green roof. Plants reduced the amount of heat absorbed by the growing medium during daytime through shading and reduced the surface temperature of the green roof. Ceiling temperatures of rooms under the existing roof and green roof indicated that heat transfer to the room beneath the green roof was reduced as well. The green roof reduced the heat gain due to the thermal mass of the soil. This created a buffer against daily fluctuations in temperature and minimized temperature extremes.