Rapid urbanization has exacerbated the influence of the urban heat island effect in existing cities, particularly, in low-rise and densely-populated areas. Accordingly, small urban green spaces have been employed to lower the temperatures in urban areas and improve the quality of life. However, the current research on the quantitative analysis of the size and shape of urban green spaces, which maximize the temperature-reducing effect, is insufficient. Therefore, a quantitative analysis was performed in this study to determine the impact of the shape of an urban green space on the cooling effect of vegetation and range of impacts based on three elements: the area, perimeter, and shape index of the urban green space. Method: A computational fluid dynamics (CFD) simulation was performed and used to analyze the impact of the shape of the urban green space on the temperature-reducing effect and range of impacts by examining eighteen urban green spaces in three target areas. To this end, programs such as STAR-CCM+ and GIS were employed. Results: The analysis results show a meaningful relationship between the green space area and range of the cooling effect of the vegetation, and indicate the importance of the perimeter of the vegetation in reducing the temperature of the central and surrounding areas of the urban green space. Moreover, although the shape index is not a decisive factor, slender and intricate shapes were found to have a higher cooling effect for a short distance. Therefore, this study contributes to the establishment of a guideline for the development of a pleasant environment in existing cities.