This study reports the effects of wood fibers and 3 wt% maleic anhydride-grafted polypropylene used as coupling agent on the tensile properties of polypropylene/wood composites. Compounding was done in a roller-based internal batch mixer followed by compression molding. Our findings show that both birch and aspen wood fibers improve the elastic modulus and the tensile strength of composites, and the chemical treatment improves the fiber–matrix interface. A comparison of experimental results’ elastic modulus with micromechanics theoretical models shows that the Lavengood–Goettler model is closer to experimental data. Also the results showed that the polypropylene/wood composites’ elastic modulus exceeds high-performance thermoplastics commonly used in gears manufacturing. Thus, the price of polypropylene/wood fibers makes it a viable alternative for similar application.