A nanoparticulate-hydroxyapatite filler augmented osteointegration within a resorbable polymer based bone graft substitute designed for orthopaedic and periodontal applications. The unsaturated polyester poly(propylene glycol-co-fumaric acid) (PPF), was used to prepare the bone graft substitute. The nanoparticulate-hydroxyapatite filler was examined in terms of biocompatibility, bony ingrowth and mechanical stability in a rat calvarial defect model. The nano-hydroxyapatite fillerwas compared against a commercially available micrometer-sized hydroxyapatite(HA) filler. Histological analysis indicated that remodeling of the newly formed bone was more advanced in the defect filled with the nano-hydroxyapatite augmented PPF. Mechanical evaluation showed a more rapid increase in stiffness of the nano-hydroxyapatite PPF. Implants of the nano-HA augmented PPF showed more advanced bone formation and recovery of mechanical properties, suggesting an improved biological response to the nano-sized particles.