It is well known that for polymer composites based on natural fibers, the addition of a coupling is necessary to improve fiber dispersion and adhesion with the matrix. Nevertheless, an optimum content must be found which is related to the total surface area created between the fibers and the matrix. But in most cases reported in the literature, a single property (like tensile strength or flexural modulus) is used to determine this optimum value. In this work, high density polyethylene (HDPE) was reinforced with hemp fibers as a typical system. In particular, the addition of a coupling agent based on maleated polyethylene (MAPE) was studied to determine its optimum content. To better detect the specific effect of the selected coupling agent, reinforcement content was limited to 10% wt., while the MAPE content was controlled at different levels (0, 5, 7, 9 and 11% wt. based on total hemp content). Compounding was performed in a twin-screw extruder and the samples were produced by compression molding. From the composites obtained, a complete characterization in terms of morphology (SEM), mechanical properties (tension, torsion, flexion and impact), and density (pycnometry) was made. From all the results obtained, it was shown that 9% wt. MAPE was the optimum content to maximize all the mechanical properties (including density), with the only exception being the notched Charpy impact strength. These results indicate that any physical property besides impact strength can be used to determine the optimum coupling agent content.