The influence of matrix modification on the interfacial shear strength (IFSS) and the mechanical performance of polypropylene/glass fibre composites is investigated. Two different modifiers were used: a highly reactive hyperbranched polymer grafted polypropylene (HBPgPP) and a maleic anhydride grafted polypropylene (MAHgPP). The interfacial shear strength increased with the addition of the modifiers, with HBPgPP giving the highest values. To evaluate the effects of the matrix modification on the composite strength, a method to normalise the composite strength with respect to fibre orientation and fibre concentration is presented. The normalised strength values followed the same trend as the measured IFSS values, namely that the HBPgPP modified composite displayed the highest strength and the unmodified material the lowest.