A study on granularity distribution of airborne microbes was conducted in details in summer, and the fluctuation regular was analyzed in Beijing. Results show that the distribution characteristics which are not changed with the different functional regions and periods are very different among airborne bacteria, fungi and actinomycete. Airborne bacteria appear skew distribution, the particles larger than 2.0 microm account for 80.0% of the total while smaller than 1.0 microm contribute 9.0% approximately. Airborne fungi are recorded with normal logarithm distribution, the particles between 1.0 microm and 6.0 microm account for 70.0% while smaller than 1.0 microm contribute 5.0% approximately. The distribution of airborne actinomycete are completely opposite to fungi, the particles larger than 8.2 microm and smaller than 1.0 microm account for 60.0% while between 3.0 microm and 6.0 microm contribute 10.0% approximately. Moreover, the granularity distribution of dominant fungi is consistent in different functional regions. But Cladosporium, Penicillium and Aspergillus are most collected in F3, F4 and F5 grades (1.0-6.0 microm) while Alternaria and nonsporing most in former four grades (>2.0 microm) of FA-I sampler, contributing 85.0%, 85.0%, 85.0%, 90.0% and 75.0% of the total respectively. The granularity of former presents normal logarithm distribution and latter skew distribution. In the past ten years, it is no change on the trend of microbial granularity distribution in Beijing, but the peak value declines from 3.0-6.0 microm to 2.0-3.0 microm.