An experiment was conducted to study the effects of storage layer and plant selection on the evapotranspiration of green roofs quantitatively. By setting 4 pilot green roofs designed with different structures and different vegetation, the variation of evapotranspiration (ET), soil moisture content and the corresponding weather conditions were monitored and recorded. The results show that, for both of the green roofs vegetated with Sedum lineare and Callisia repens, the water evaporated from the storage layer plays the role in the recharge of the moisture content of the upper soil layer. In addition, it shows a positive correlation between the initial soil moisture content and daily evapotranspiration in varying degrees during drought periods. The setting of storage layer would increase the evapotranspiration of a green roof. However, the correlation level of the green roof vegetated with Sedum lineare is lower than that with Callisia repens, due to its complexity and particularity of the crassulacean acid metabolism (CAM) mode, and the hourly evapotranspiration fluctuates greatly. Different from Callisia repens, the stomas would close when the solar radiation is very strong to reduce the amount of transpiration and retain water. It would provide a theoretical basis for the assessment of ecological benefits on evapotranspiration of green roofs.