1. The need to understand the mechanisms enhancing species coexistence and diversity is one of the central issues of community ecology. Theoretical work suggests that resource partitioning based on body size contributes to the coexistence of competing species, resulting in increased species diversity. Nevertheless, few empirical studies have investigated this. The species composition of wood-dwelling arthropods was surveyed to examine resource partitioning based on body size in the dead wood of the ubame oak (Quercus phillyraeoides A. Grey) trees.2. Greater resource sizes (larger dead wood diameter) retained a lower overall density of wood-dwelling beetles per resource volume, greater total beetle biomass per resource volume and larger bodied individuals of the dominant species (Cerambycidae; Anaglyptus nipponensis Bates). A gradient in the species composition of arboreal ant assemblages along resource size and a positive correlation between resource size and ant body size was observed. These results suggest that the incidence of wood-boring beetles was limited by the size of the available resource, resulting in resource partitioning by the ants according to the sizes of galleries made by the beetles.3. Our findings indicate that resource size leads both directly and indirectly to body size-dependent resource partitioning in beetle and ant assemblages, and contributes to the maintenance of species coexistence in the assemblages present in dead wood.