In this study, we evaluated the influence of wood flour (particle size: < 90 mu m) on the physical properties of wood flour/polypropylene (PP) composites. Wood flour was obtained by dry ball-milling of forest-thinning material at a rotary speed of 250 rpm for 1, 2, 4, and 8 h. The milled wood flour was filtered using a 90 mu m sieve. The water content of the wood flour was adjusted to 5, 10, or 20 wt%. Composite properties, including mechanical properties, water absorption, and thermal expansion, were evaluated at wood flour loadings of 40 wt%. When wood flour with 5 wt% water content was milled for 2 and 4 h, the resulting wood flour was granular rather than fibrous; flocculation of the fine particles was observed for milling times exceeding 4 h. This morphological change in the wood flour reduced its influence on the physical properties of the composites, although some positive influences were observed on the molding properties of the composite, such as an increase in compound fluidity. Milled wood flour with 10 wt% or 20 wt% water content was fibrous. Scanning electron microscopy observation of milled wood flour with 10 wt% water content revealed partial surface fibrillation at widths of tens to hundreds of nanometers. The addition of wood flour with nanoscale surface fibrils to PP composites positively influenced the properties of the composite, resulting in a decrease in the linear coefficient of thermal expansion in the flow direction.