Control particleboards were significantly higher in thickness swelling than wood particle-polypropylene fiber composites and their thickness swelling increased with the increase of target density. In the composites, thickness swelling did not vary significantly with the increase of target density but increased with the increase of wood particle content. And the composites with fine wood particles were significantly lower in thickness swelling than those with coarse wood particles irrespective of target density and formulation. In the scanning electron microscopy, significantly higher thickness swelling in the composites with coarse wood particles was thought to be the result of more interfacial separations by higher swelling stresses.