Polyethylene-kaolin composites were investigated with a special emphasis on the control of the interfacial adhesion. Both matrix and filler were modified for this purpose. A stearic acid and maleic anhydride-grafted polyethylene were used as potential interfacial agents and the efficiency of aminosilane-surface-treated kaolin was considered. Tensile strength, elongation, impact strength, and melt index were currently measured in relation to the processing conditions. Enhanced interfacial filler-polymer adhesion progressively results in an decreased melt index. This has been clearly shown by comparing the effect of two polymeric additives to the polymer matrix, i.e., a maleic anhydride-grafted HDPE (MAGPE) and an unmodified HDPE of a similar melt index. Compared to low molecular weight additives, such as stearic acid and aminosilane, MAGPE has proved to be a very efficient additive in improving the impact resistance of HDPE-kaolin composites even at low contents.