AbstractBark, as a residue from trees, is mostly used for thermal energy production, but a better utilization of this resource was considered as an alternative raw material for wood-plastic composites (WPCs). The influence of bark, wood, and blending of bark and wood flour content of the poplar tree on the mechanical characteristics of WPCs were investigated. Wood and bark flours with 2% maleic anhydride-grafted polypropylene (MAPP) and polypropylene were compounded into pellets using a counter-rotating twin-screw extruder, and test specimens were prepared by injection molding. The results showed that both bark fiber and wood flour increased mechanical strength (flexural strength (MOR), flexural modulus (MOE), tensile modulus, and tensile strength) significantly (P<0.05). Composites made with bark flour exhibited lower mechanical strength compared to those made with wood flour and wood flour/bark flour. Differences in chemical composition between bark and wood, fines, low aspect ratio (length/width) of bark flour, delamination between fines and matrix, and the lower intrinsic fiber strength of bark fibers compared to wood fibers are good explanations for this demarcation. The notched impact strength of all reinforced composites was significantly lower than neat polypropylene (P < 0.05).