The purpose of this study is to investigate and present a method for measuring public accessibility to urban open spaces. A basic assumption is that, for urban open space policies, accessibility is more important than per capita area. In this study, for the purpose of simplicity, a residential area is assumed to have access to open space if it is within a certain distance from an urban open space. Official city planning map is overlayed with a 200m grid and each cell of dwelling area is checked whether it is within a certain distance from a cell categorized as urban open space. A computer program for widely commercialized personal computer is developed for data processing so that local governments without access to more sophisticated systems can carry out similar studies for their own jurisdictions. Five cities, big, small, old and new, are selected to test the proposed method. Dwelling areas of Ansan new Town have highest accessibility to open spaces(93.4% of dwelling cells have open space cell within 500m). Seoul (91.2%), Suwon(78.2%), Pusan(73.8%), and Inchon(61.4%) have less accessibility. If we assume the Ansan City residents are evenly distributed over the dwelling area, 93.4% of the population has open spaces within walking distance of 500m. However, if we consider physical barriers such as arterial roads, railroads, and streams that reduce the accessibility, less than 93.4% of Ansan city residents enjoy good access to open spaces. Though a further detailed analysis is needed to picture the microscopic accessibility, this method can serve as a useful tool for urban open space policy and open space alternatives evaluations.