Mandibular fractures present a challenge in maxillo-facial surgery due to difficulties in healing and complications. In recent years, advances in bio-engineering as well as stem-cell studies suggest that it may be possible to treat these fractures by stem cell treatment with biomaterials. In the present study, we explored the efficacy of differentiated stem cells placed on biomaterials on fracture treatment and its relation with oxidative stress and apoptosis. A 4 mm circular defect was made on the mandibulae of 20 adults Wistar rats. Hydroxyapatite gel (control, n=5) and bone marrow stromal cells differentiated into osteoblast-seeded hydroxyapatite gel (n=5) were implanted within these defects. We were also used empty cavities (n=5) and cavities filled with only cells (n=5) for negative controls. Animals were sacrificed after a 6-week healing period and samples were examined blindly by histological, immunohistochemical, radiological and morphometric methods. Compared to the control cavities that underwent no procedure or filled with just cells, there were significant (P<0.001) healings in both groups. Hydroxyapatite gel with differentiated stem cells on, however, yielded significantly (P<0.05) better new bone formation and osteoid production decreased fibrous tissue and increased cellular activity. Differentiated stromal cells combined with biomaterial accelerated the treatment in defects of critical volume within a 6-week period of healing, activated and resulted in significant formation of bone of higher quality. Promotion of bone formation by the helps of bioengineering and stromal cells has gained importance in the treatment and reconstruction of fractures.