The purpose of this study is to evaluate the role of decortication of cortical bone on bone formationin periosteal distraction osteogenesis . Titanium mesh was placed between the periosteum and lateral surface of the mandible in 10 adult rabbits. The buccal cortical bone was porously perforated by drilling in 5 rabbits ( test group) and the others without decortication ( control group). Rabbits were sacrificed after 8 weeks. Specimens were fixed, decalcified, and stained with hematoxylin and eosin. Histologic examination was performed on all specimens. Histologic evaluation in control group showed a newly formed bone under the titanium mesh, large marrow spaces were observed in the regenerated region