In order to the mitigation effect on urban heated environment of urban park, the microclimate observations have been carried out in an urban park and its surrounding in Tokyo since 1998. We have installed the 17m height observation tower with the sensors to measure air temperature and humidity at each 2m within the forest during 1st-30th July 2005. And also 19 measurement systems for air temperature and humidity were installed inside and outside the forest. As a result, the existence of clearly high- temperature and humidity layers were caught in canopy layer during the daytime. This means the exchanges of latent and sensible heat were very active in the canopy layer. In night, specific humidity near the ground was higher than that of upper layer because of continued evaporation from the ground surface. Moreover, when the flux of solar radiation was less than 300W/m² (e.g. cloudy day and nighttime), air temperature inside the forest was lower than outside the forest; maximum difference was 5 degrees Celsius. However, there was stark difference air temperature between inside and outside the forest during the sunshine daytime. We can not clearly identify the mitigation effect on urban heated environment of urban park in this field observation.