Micro-climatological observations were performed in and around large green space "Shinjyuku Gyoen" in Tokyo during summer. Green area is always cooler than surrounding residential and commercial area, at most 3 degrees. In daytime windy condition, cold air mass from green space chilled the leeward built-up area, on the contrary, warm air advection from windward built-up area into the green space was also existed. In order to catch seeping-out phenomena of cool air in calm condition, four 3-dimensional ultrasonic anemometer-thermometers were set along the boundary. In clear calm night, flow-out wind direction from green space to surrounding area were discerned at all measuring points. During this condition, significant air temperature drop were observed within the range of 80-90m area from the boundary. According to the eddy correlation measurement, sensible heat flux at lawn surface was about 100W/m2 in mid-day, which amounts to 15% of net radiation. It was changed to negative immediately after sunset, then lawn surface was acted as a heat sink all the night through. Air temperature above lawn surface was lower than that of under tree crown during night. This temperature difference was diminished by intermittent wind and cloud cover.