The effect of grafting level of maleic anhydride (MA) in the maleated polypropylene (PPMA) on the fracture, deformation mechanisms, and mechanical properties of polypropylene (PP) wood flour composites was studied. Tensile strength, elongation at break, and impact strength are noticeably improved with addition of interfacial modifiers as maximum values of the examined mechanical properties were detected when concentration of MA in the compatibilizer was 1 wt %. To explore the microstructure and deformation mechanisms, scanning electron microscopy was employed. It was found that low concentrations of MA up to I wt % led to the creation of a thin and irregular polymer layer assisted formation of fibrillated plastic deformation zone around the wood particles, while the bulk PP matrix experienced voiding and brittle fracture. Higher concentrations of MA fetch to stronger interaction between PP and wood flour, the reason for brittle fracture and reduced ductility of the matrix. The impact fracture behavior of the composites during Instrumented impact tests is also discussed with respect to the interfacial bond strength. (C) 2004 Wiley Periodicals, Inc.