The problem arising from limb lengthening is rather common and numerous methods have been devised. However, the clinical application of limb lengthening techniques have been increasing and have attracted many attentions. But, the mechanism by which lengthening is achieved has not been clearly identified, yet. The present study was undertaken to compare the effect of latent period and osteotomy method on the callus distraction of the rabbit tibia. Experimentally, the authors have performed lengthening of tibia in 15 growing rabbits using dynamic unilateral fixator which was designed by Department of Orthopedic Surgery, Chungnam National university Hosptial. The animals were then divided into four experimental groups. The group I received complete osteotomy and started distraction from the operative day at the rate of 0.25mm/12hours. The group II received complete osteotomy and started distraction from the postoperative 7th. day at the same rate. The group III received corticotomy and started distraction from the operative day at the same rate. The group IV received corticotomy and started distraction from the postoperative 7th. day at the same rate. We checked X-ray and sacrificed the rabbits of each group weekly and performed microangiography and histological study (H-E stain and Azan-Mallory stain). The results were as follows. l. In radiographic study, the callus formation was observed most abundantly in the group IV, which is corticotomy plus 7 days latent period. The group I which is osteotomy plus immediate distraction showed least amount of callus formation. 2. Im microagioraphic study, the more preservation of medullary vessel continuity was achieved successfully in the group IV with 7 days latent period and corticotomy. 3. In histologic study, intramembranous ossification arranged perpendicular to longitudinal axis of distraction was noted in the interzone quite similar with growth plate. New bone contained islands of cartilage masses suggesting presence of enchondral ossification in area of external callus. In conclusion, we could postulate that the mechanism of the limb lengthening involves both enchondral and intramembranous ossification. Endosteum and bone marrow seem to be indispensable for adequate callus formation. Periosteum plays an important role during distraction. Consequently, preservation of bone marrow and medullary vessels seems to be important in limb lengthening procedure.