As a result of the growing population and the increasing extremes of Urban Heat Island, the value of urban green spaces is getting more recognized. These green spaces - especially urban parks - provide numerous ecosystem services to the local citizens creating a more livable environment. Many studies state that accessibility to these services has a key role in the realization of these positive effects, thus accessibility assessment has got more attention during the recent years in the field of ecosystem services. In this paper walking travel-time based isochrone maps and street resolution population data (number of local inhabitants) were used to estimate the number of city dwellers within the accessibility zones. This study is expected to provide a new method for measuring green space accessibility and help urban planners and decision makers understand the spatial differences between the various urban parks in a Hungarian case study area (Zalaegerszeg city).