To clarify the adjusting effects of urban forest canopy on microclimate,a Decagon micro-meteorological monitoring system was used to conduct long-term monitoring on illumination, temperature, relative humidity and other microclimate factors of urban forest canopy in Nanjing. In contrast to the experimental observations outside the forest, the microclimate characters inside the urban forest canopy and the effect of forest canopy on adjusting microclimate were analyzed. The results showed that during growing season,the solar radiation inside forest canopy fluctuated along a bimodal curve or multi peak curve. Seasonal solar radiation difference between the inside and outside of the forest canopy was in descending order of summer,autumn,spring and winter. The daily variations in temperature and relative humidity inside the forest canopy were similar to that outside the canopy, showing a parabolic change. The forest canopy exerted a greater cooling effect in summer and autumn than in spring and winter ; during spring,summer and autumn,the temperature difference between the inside and outside was large in the daytime, but small at night. The relative humidity inside the forest canopy at any time during spring, summer and autumn was unexceptionally higher than that of the outside. In winter, the relative humidity inside the forest canopy was lower than that of the outside after noon. The forest canopy performed better in moisture preservation in summer and autumn than in spring and winter. The advent of maximum temperature inside the canopy was about 30 minutes later than that of the outside. Except that there was no significant correlation between solar radiation inside the forest canopy and the temperature outside, all meteorological factors were significantly correlated each other (P<0.01). Collectively, urban forest canopy plays a significant role in adjusting microclimate and improving comfortable level of climate,for example,reducing solar radiation,cooling and humidification, mitigating the change in temperature and humidity inside the forest canopy and delaying the advent of maximum temperature.