Sustainable development is considered the main factor for setting eco- cities. Different interactive relationships between the main elements for sustainable development and the actual situation of the different cities were taken into consideration at this study. The main objective of sustainable development is, how to meet and achieve the required standard specifications of eco –cities in urban planning. A case study was conducted for two cities, the first one was Masdar city in Abu-Dhabi, Emirates and the second was Gangneung city in South Korea. This study discusses the methodology of turning the existing cities to green cities by applying the main requirements and standards of the green cities. These requirements are based on physical, economical, social and environmental aspects. Turning the existing cities into green cities may be achieved by maintaining and developing the available natural limits and resources. Economical studies are based on different factors such as, standard of living and land values. Social studies included the size of community, rates of unemployment and illiteracy. The environmental studies included water resources, renewable energies and natural topography of the concerned cities.