The purpose of this study is to investigate the effect of wood particle size on mechanical properties of thermoplastic composites.The target of wood particle size is 100 tun or less.Wood chips of cedar and cypress were employed for the feedstocks of wood flour.The particle size of wood flour was prepared to three types by wet ball milling.Polypropylene and maleic anhydride grafted polypropylene were used for the matrix and coupling agent of the composites.The wood flour and the maleic anhydride grafted polypropylene were contained at 10 wt% in the composites.The test specimens were prepared by injection molding.The mechanical properties are evaluated with the uni-axial tensile test and the nano indenter.These results indicate an obvious particle size effect on mechanical properties,which is caused by mechanical property change of matrix.