<p>Green Open Spaces (GOS) have a variety of ecosystem services which can be utilized by society. This study aims to identify criteria for determining ecosystem services in GOS, to study the conditions and distributions of potential ecosystem services in the Urban Area of Yogyakarta (UAY)’s GOS, and optimize the existing ecosystem services in GOS. The research’s locations include GOS in UAY consisting of urban forests, city parks, riverbanks, and greenbelts, for a total of twelve research locations. The identification of the criteria for GOS was accomplished with the expert opinion and comparative methods resulting in weight values. The field data and expert assessments were analyzed through a scoring analysis to quantify the level of ecosystem services in each research location. This data was further analyzed by examining the area allotments within the existing regulatory framework to the determine the conditions of GOS with optimal ecosystem services. The results of the study show that each of the sixteen examined criteria plays a role in ecosystem services. The values of ecosystem services of GOS in UAY show different values for each type of GOS. In general, the ecosystem services in urban forests are rated favourably in each category, while city parks and riverbanks moderate ratings, and greenbelts have the lowest average performance of any GOS. Based on the results, optimizing ecosystem services in UAY’s GOS can be accomplished by strengthening their performance in each criteri<span lang="IN" style="">on</span> with the most attention paid to each GOS’s allotment type.<br><b> <!--[if !supportLineBreakNewLine]--><br> <!--[endif]--></b></p>