The school grounds should provide a well designed and adequately sized playing fields, schoolyard as well as adequate parking, circulation, informal and social areas. Unfortunately all too many of the existing schools and some of the newest schools, particularly in urban developing sites and other densely settled areas, provide neither a sufficient area for school yards or parking. Even when available areas are ample, schoolyard design is often perfunctory, with little attention given to the research or best practices on schoolyard design. This study aims to propose the optimum size of school ground especially built in urban developing sites. The functions and needs of school ground have been changing from the areas for physical education and training to for various activities such as recreation, play, communication, informal and social areas etc. Therefore, the size also should be enlarged and well accommodated a variety of needs to school ground. In doing so, it surveys the school ground size of the existing schools and the newest schools built in Dongtan City and categorizes them into 6 elements : playing field, court yard(Ma-dang), habitat area, building area, parking lot, road and access. Finally, it proposes an adequate Size of School Ground in Urban Developing Sites. This study would help to plan the optimum size of school area and ground and the reasonable expenditure for school facilities.