The scope of the present paper is to study the effect of adding a novel coupling agent, polybutadiene isocyanate (PBNCO), on the mechanical properties of hardwood aspen fiber/polypropylene (PP) composites. The resulting properties were compared to those obtained with the most commonly used coupling agent, maleic anhydride modified polypropylene (MAPP). In this study, we determined a value of 24.75 MPa for the tensile strength of pure PP and 22 MPa for the composite containing 30 or 40wt% unmodified fibers. These results indicate that wood fiber behaves merely as filler when incorporated into PP and no reinforcing effect was observed in this case. This occurs because of the chemical incompatibility between the thermoplastic PP and the polar fiber, resulting in low interfacial adhesion. However, it was verified that addition of 3% MAPP and 5% PBNCO to this formulation produced composites with better performance, since the tensile and impact properties were increased up to 30 MPa and 22 J/m², respectively. This behavior can beattributed to the enhanced interfacial bond between reinforcing fibers and polymer matrix modified MAPP and PBNCO treatments, play a significant role in improving the mechanical properties of the composites. The increase in mechanical properties demonstrated that PBNCO is an effective coupling agent for wood fiber/PP composites.