In large cities such as Osaka which are located in the southwest part of Japan, the urban heat island prevents peoples from deep sleeping in summer without using an air conditioner. Today measures to decrease an urban heat island are hoped in these cities. It is well known that the air temperature under the large green canopy in a park is lower than the surrounding urban areas where buildings are concentrated. We called the green space �gCool Spot�h. One of the effective measures to decrease the local air temperature will be covering an area with many Cool Spots. In order to investigate the relation between the cool spot effect and the size of cool spot, some sets of meteorological elements of air temperature, ground surface temperature and the others were measured in and around urban four green canopies in Osaka, for four days in August 1992. The cool spots are two parks and two green areas besides streets. The size of four cool spots is between tens to hundreds meters. The relation of the temperature difference between inside and outside of the cool spot, and the length of cool spot, wind speed, the ground surface temperature and eddy diffusivity were examined. Finally a simple model was proposed to predict the cool spot effect. The predicted values of temperature by using the model have good correlation with the observed values of temperature.