Thermal performance of the green roof is an important basis for energy-efficient design and evaluation. However, the current studies on the thermal performance of the green roof seldom involve thermal inertia of the green roof. Based on a long-term experiment of an extensive green roof in natural ventilation state and a five-month ongoing measured data in summer, thermal inertia of the extensive green roof was studied by statistical analysis method. The results show that thermal decay to outdoor climate effect through green roof was increased by more than 100% and delay time of thermal response to outdoor climate on internal surface of the green roof was reduced in comparison with that of the bare roof.