This study evaluated the individual and interaction effects of inner bark flour (IBF), outer bark flour (OBF), wood flour (WF), and blending of IBF, OBF, and WF content of poplar tree on the morphology and mechanical properties of wood-plastic composites (WPCs). The IBF, OBF, and WF with 2 wt% maleic anhydride-grafted polypropylene (MAPP) and polypropylene were compounded into the pellets using a counter-rotating twin-screw extruder. Test specimens were prepared by injection molding machine. The results indicated that the WF alone significantly (P<0.05) increased the flexural strength, flexural modulus, and tensile strength. The composites made with IBF/WF exhibited higher tensile modulus compared to those made with WF alone and IBF/OBF alone. Moreover, the results showed that the IBF/OBF alone increased the notched impact strength compared to all of reinforced composites. The neat polypropylene had higher notched impact strength than the other reinforced composites (P<0.05).