It was researched the structure of downy oak tree (Quercus pubescens Wild) arboretum in relation to the diameter which being represented by the oak frequency distribution curves on diameter categories shows competition relations among the individuals in populations. The obtained results reveal that vigorously growing trees gradually occupy positions more and more favorable compared with neighboring individuals. It allows them over time to form thicker diameter trunks by accumulating woody biomass. Such growing relationship of individuals determines the part of the right distribution curve, where larger diameters of downy oak trees are located, to elongate in the detriment of its left part forming a left positive asymmetry.