Appropriate microclimate design for urban parks is important, as it impacts on thermal comfort and thereby influences the utilization of outdoor spaces by the public. Modifying the vegetation arrangement can improve the microclimate, especially under hot season conditions. However, it is difficult to measure microclimatic variations in the individual components within a park. Therefore, we combined in-situ measurements and a three-dimensional microclimate model ENVI-met, to examine variations in the microclimate of an urban neighborhood park in Seoul, Korea. Different vegetation arrangements were investigated with respect to their effects on microclimate and thermal comfort on a typical summer day. The study showed that: 1) the simulated results of ENVI-met were similar to the observed data, with an index of agreement of 0.69–0.88; 2) green spaces with multi-layered and dense canopy cover provided the most comfortable conditions. However, the cooling efficiency of vegetation may decrease when the plantation is too dense; and 3) the main factor affecting thermal comfort in the hot season was mean radiant temperature. The findings of this study provide useful information to guide planning of vegetation arrangements in future urban park designs.