Roof planting is widely recognized as a ecological thermal insulation for roof. A lightweight green roof in climatic conditions of shanghai was investigated as a case study by applying PMV index to calculate thermal comfort compared with that with traditional roof in air conditioning. It is found that the PMV value of green roof is 0.2 lower than that of traditional roof, while the fluctuating range of green roof was only 1/4 of that of traditional roof. And the equivalent thermal insulating roof of green roof was obtained by computing. Furthermore, it is shown that green roof has lower interior surface temperature than equivalent insulating roof in the condition of higher indoor temperature.