AbstractBio-fibers have been used for some time to reinforce thermoplastic composites; such structures are being used in a variety of commercial applications. In this study, wood fiber and flax fiber were used to reinforce high-density polyethylene (HDPE) formed by extrusion. The flexural, tensile, and impact resistance properties of the resulting flax fiber/wood fiber/HDPE (F/W/HDPE) composites were measured and modeled as a function of the volume fraction of flax ﬁber. Finally, the correctness of the modified model was verified. Based on the measurement data, the volume fraction of flax ﬁber was shown to play an important role in determining the mechanical properties of these composites. With increasing flax fiber volume fraction, the flexural strength, tensile strength, tensile modulus of elasticity, and impact resistance of the composites generally increased. However, the flexural modulus decreased. Based on the rule of mixtures (ROM) model, two coefficients were introduced and a new curve-fitting model was established based on measurements of macrostructure. Compared with the traditional ROM model, the new model developed in the present study could describe the flexural strength, tensile modulus, and impact strength of F/W/HDPE composites more accurately.