membrane, insulation layer and structural layer. And the substrates and plants of green roofs are greatly different from that of ground greenery. The substrate is usually engineered lightweight soils,which has large water holding ability and permeability. Its major components are aggregate (a porous lightweight materials), sand and organic matter , which depend on local availability and cost of materials. Plants selected for green roofs should often meet some requirements, including excellent drought tolerance, low grow thrate and fast establishment etc. The most commonly used plant sworldwide for extensive green roofs are sedum species, they belong to succulent plants and have unique CAM2photosynthesis that can reduces water loss, and exhibit strong adaptability to various climates. Other species recommended for extensive green roofs includes Euphorbiaceae and Portulacaceae. It is suggested that mixed species have advantages over single species.