This article deals with the assessment of the influence of technical, processing, and material factors on selected mechanical properties and the granulometric composition of juvenile poplar wood chips. Individual analyses were made for two poplar species: naturally grown Populus tremula L. and the cultivated poplar clone Populus x euramericana “Serotina” for both juvenile and more mature wood. The main goal of this study was to evaluate the influence of the selected technical and processing parameters and of wood type (juvenile and more mature) on the chips’ granulometric composition during plane milling, its granulometric composition, and the sizes of the greatest and the smallest particles. Granulometric analysis (size test) was carried out to determine the share of the grain sizes for the individual wood fractions. While evaluating the granulometry, the influence of the milling process conditions as well as that of the wood’s physical and mechanical properties was taken into account.