In this article, a commercial grade of polypropylene (PP) was reinforced with short poly(ethylene terephthalate) (PET) fibres up to 30 wt%. In order to have a reactive interface, two different compatibilizers, glycidyl methacrylate (GMA)-grafted PP and maleic anhydride (MA)-grafted PP, were used. To choose an optimized fibre length for the PET fibre in PP/PET composites, critical PET fibre length was evaluated applying stress analysis and Von Mises yield criterion. PP/PET fibre pre-pegs were prepared by melt impregnation technique using an internal mixer followed by the compression moulding. Tensile and flexural properties were investigated. Morphological studies were carried out by scanning electron microscopy technique. It was observed that the addition of PET fibres enhanced flexural modulus compared with that of pure PP, which was higher for GMA-modified composites. The tensile modulus results also showed enhancement by the addition of PET fibres. However, the tensile modulus of unmodified specimens versus fibre load was little greater than that of unmodified samples up to 15 wt% of fibres due to higher crystallinity. The flexural strength results of modified composites versus fibre wt% also showed enhancement that was higher for those of GMA-modified ones. The fibre to matrix bonding was better in the presence of GMA than that of MA, as revealed by scanning electron micrographs of tensile fracture surfaces. Halpin–Tsai, Pan and Cox–Krenchel equations were applied to predict the tensile modulus of random PET fibre-reinforced PP composites, among which the Pan’s model had the best prediction. To evaluate relative reinforcement at different fibre loads, fibre efficiency factor was calculated for GMA-modified composites.