Wood plastic composites (WPC) are composite materials which consist of wood flour and thermoplastics.WPC have been applied to building materials and automobile components since 1990.The range of wood particle size which is used in WPC is generally between 300 μm and 1 mm.In contrast,many recent studies have focused on composites using lignocellulose nanofibers which are obtained from wood.However,it is costly to fabricate lignocellulose nanofibers composites in comparison to WPC.These composites are required to have more superior properties such as low-cost and functionality.The purpose of this study is to investigate effects of wood particle size on mechanical properties of WPC.The target of the wood particle size was 100 μm or less.The wood chips obtained from thinnings such as cedars and cypresses were employed in this study.The wood flour was prepared by wet ball milling.The average particle size of the wood flour was controlled by changing the milling time,was adjusted to three types (approximately 30 μm,60 μm and 140 μm).In the heating and kneading process,the compounds were prepared by mixing the wood flour of 10 wt% with polypropylene (PP) of 80 wt% and maleic anhydride grafted polypropylene (MAPP) of 10 wt% as coupling agent.The composites were prepared by injection molding the compounds.