This paper aims to investigate the effects of tree communities on the improvement of microclimatic and human comfort conditions in urban areas, specifically, in the case of Beijing Olympic Forestry Park in Beijing, China. The microclimate characteristics of different communities were analyzed，and the discomfort index（DI）was introduced to evaluate the effects of different tree communities on human bodys comfortable degree. The results indicated that there existed significant differences in the air temperature, relative humidity and light intensity between tree communities and the control open site（CK）. Compared with CK, the tree communities can daily decrease temperature by 1.6-2.5 ℃，increase the relative humidity by 2.9%-5.2%, and the shading rate was 61.0%-96.9%. Compared with CK, all species communities can reduce the diurnal mean DI in some degree and the decreasing rate was 2.5%-4.3%. Correlation analysis between microclimate factors and the indices of tree community canopy structural characteristics showed that canopy characteristics played important regulatory role in microclimate and DI degree. The intention of this work is to provide more basic and solid information to aid human to understand the role of plant on mitigation of the destroyed environment, and specifically, to aid efforts to improve the environment of Beijing through better planning and the appropriate choice of plant species used for landscape design,and for references of other cities having similar climatic characteristics as well.