Vascularized periosteal flap(VPF) it thought to enable formation of new bone and promote union in bone defect without risk of complications in donor site. Studiol about the VPF thus far have been centered on the long term result than early change after YPF. The purpose of this study was to elucidate the process of new bone formation in early stage after YPF by radiological, histological, electron microscopical and immunohistochemical studies. In experimental group, segmental resection of bone including the periosteum was performed in fifty-six radii of immature New Zealand white rabbits preserving the periosteal circulation from median artery which is the main source of blood supply to the periosteum. In order to simulate the transplantation of VPF, the vascular pedicle of median artery and veins was dissected from adjacent soft tissue and the periosteum was longitudinally incised to remove the bone tissue in the periosteum. Thereafter the vascularized periosteum was repaired. From the first to fourteenth day after the simulated VPF, the findings in the VPFs were observed by radiological, light microscopical, scanning electron microscopical methods and activity of osteocalcin was measured by immunohistochemical method. In control group, the bone tissue and periosteum were completely removed from the mid-shaft of seven radii, thereafter the radiological findings were observed at 1, 2, 3, 4, 8, 12, 16th week and light microscopical findings were observed at 8, 16th week after operation. From the results of this study, it is concluded that YPF is vigorously and uniformly osteogenic in its early stage. It it thought that this study could serve as a basic data for the future experimental studiol about VPF in animals and clinical application.