The fracture behaviour of glass microsphere-filled polypropylene/poly(ethylene terephthalate-co-isophthalate) blend-matrix composites was investigated in comparison with that of the glass microsphere-filled PP composites. Depending on the deformability displayed by the composite, it was carried out through the linear-elastic fracture mechanics or by applying the J-integral concept. The matrix ductility was regulated in the composite through the glass bead surface treatment applied with different silane-coupling agents, as well as with the addition of maleated PP as polymer compatibilizer. Whereas all the composites failed in a brittle manner at moderate impact speed, quasi-brittle fracture behaviour was only observed at low strain rate in composites having high and medium interfacial adhesion level. Results showed that composites containing both aminosilane-treated glass microspheres and maleated PP showed the highest values of fracture toughness. In composites with low adhesion level between matrix and glass beads, the critical J-integral value diminished due to the presence of PET.