In this study, rice-straw (RS) filled high density polyethylene (HDPE) composites were manufactured by extrusion and injection molding. Three compatibilizers, which are unfunctionalized ethylene/propylene copolymer (uEPR), maleic anhydride grafted EPR (EPR-g-MA) and PE-g-MA, and their combinations were introduced to strengthen fiber-matrix interphase. The mechanical and morphological properties of composites were investigated. For single-compatibilizer system, PE-g-MA or EPR-g-MA alone enhanced tensile, flexural, and impact strengths of resultant composites compared with HDPE/RS system without compatibilizers. Different toughening origins of individual compatibilizer were discussed based on composites' interphase morphologies and mechanical properties. For combined-compatibilizers system, the PE-g-MA/EPR weight ratio is important for several properties of composites. The optimum ratio was considered as 2 : 1 and 1 : 1 for PE-g-MA/uEPR and PE-g-MA/EPR-g-MA modified composites, respectively. Also, composites modified by combined PE-g-MA/EPR-g-MA showed better impact strength than that modified by PE-g-MA alone. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119:2214-2222, 2011