Many urban cities in the world are trying to enhance sustainability by improving urban greenery and promoting urban farming. By installing green roofs with urban farming, it is possible to achieve environmental, social and economic sustainability for the buildings in urban cities because it can contribute to the mitigation of environmental problems, enhancement of community functions and development of urban food systems. This paper presents the findings of a research to investigate green roof urban farming for high-density urban cities like Hong Kong. The benefits and potential of rooftop urban farming are examined; some experiences in the world are described. The characteristics and constraints of high-density urban cities are studied and the situation in Hong Kong is evaluated critically. It is hoped that the research information will be useful to promoting sustainable buildings and environment in urban cities.