It is very important to determine the size distribution of airborne microbiological particles for the purpose of engineering control of indoor air contamination. Most of the sampling apparatus of airborne microbiological particles collect them aerodynamically, and consequently the sizes obtained differ from the morphologically measured ones which are generally understood as sizes in the field of microbiology. This report is to clarify those relations between aerodynamical and morphological diameters, using Cladosporium spores of which size distribution has been known to have log-distribution with peaks around 3.5 μm through the field measurements thus far. From microscopical measurements using Porton graticule, these spores float on the air as single particles and also agglomeration of tens of spores of which maximum had 44 spores which was collected on Stage #1. The result of microscopic measurement gave the log-normal distribution with a peak at 8.9 μm which is much larger compared with aerodynamic diameter of 3.5 μm. In the same way, the histograms of each stage coincide rather well. The distribution of number of spores of each agglomerate also form a log-normal distribution cuve with median around 6-7 μm. Apparent density of spore agglomerates were obtained ; 0.13 g/m^3 for particles on Andersen sampler Stage #1, 0.41 for Stage #6. Comparison of destribution curves of actually collected particles with the theoretical ones calculated from Ranz and Wong theory and the above mentioned density was not necessarily successful and remained for further study.