We are facing a number of ecological problems in a city due to reckless urban development and damage of green areas. A roof garden has been brought to the fore as an effective and practical solution to such problems. However, a roof garden is considered to degrade durability of a building and additional load imposed on a building makes it difficult to introduce the system. Despite such problems, local self-governments as well as Seoul City are promoting roof gardens as a method of conserving green areas in a city. Moreover, the market of roof gardens is growing with such governmental support. Therefore, a new technology should be developed for roof gardens of easy installation and light materials so as to widely distribute the system. In order to meet such demand, therefore, we have developed a new technology for a green roof system. The new construction method for a green roof uses green units in a form of a box to prevent roots and to drain water. Green areas are formed by being connected with the units. This construction method makes possible fast and easy installation of a green roof. And it reduces an urban heat island by providing places for drainage, increasing the amount of evapotranspiration. This research was supported by a grant (06ConstructionCoreB02) from High-tech Urban Development Program (HUDP) funded by Ministry of Land, Transport, Maritime Affairs of Korea Government