Urban parks can help to mitigate urban heat island effects and decrease temperature of surrounding area by cooling effect. Although, various researches related to cooling effect by park have been studied, it is not clear about relationship of cooling effect with park size. Thus, Using 37 parks in Ilsan new town, Korea, this paper aims to find what does the park size occur the cooling effect and analyze the impacts of the surrounding area on park`s cooling effect. Multiple regression is used to understand the cooling effect by park size and variables are derived from landsat 8 satellite image. The results indicate that (1) the cooling effect shows on park size over 6,000. (2) park over 300,000 have a high intensity of coling effect more than other sizes. But, a maximum cooling distance of park size4~10 was similarly estimated in 180m or over 180m. We concluded that the cooling effect should be mainly considered to minimum park size with other factors. Thus, the study provides insight regarding the importance of park size for mitigation of urban heat island effect.