Recently, the necessity of the limb lengthening has been increased in the orthopaedic fields. Various methods of limb lengthening have been introduced by many authors. But there are numerous problems in the limb lengthening clinically. And the factors affect the efficacy of the distraction osteogenesis havent been fully understood yet. To find out the factors promote the new bone formation, we did the tibial lengthening procedures in 32 rabbits with mini unilateral external fixators (Mini-Orthofix). We started the distraction after one week latent period, with a rate of 0.5mm a day (0.25mm twice a day) and 2 weeks period. To compare of several factors, experimental animals were devided into three groups (I, II, III) according to the preservation of the periosteum and the medullary cavity at osteotomy site (Group I: Preservation of the periosteum & the medullary cavity, Group II : Preservation of the periosteum and curettage of the medullary cavity). Subdivided each groups into A and B (A: Subperiosteal infiltration of TGF-beta (40ng/day for 1 week), B: Subperiosteal infiltration of normal saline as a placebo during the distraction period). The regular radiological evaluation with one week interval was taken, and the histologic study (H-E & Azan-Mallory stain)and microangiographic study (by Spalteholz technique) at the lengthening site to assess the effect was done. We noted the best results such as rich callus formation, formation of zonal structure similar to the normal growth plate and restoration of marrow vessel continuity in group I (intact periosteum and medullary cavity) and the poorest results in group III(stripped periosteum and curetted medullary cavity). There was no specific difference between the TGF-beta and the placebo infiltration subgroups in these studies. Recanalization at the osteotomy site was completed at 2weeks after distraction and more prominent in group II than in group III. Pseudo-growth plate formation (Interzone) was more prominent in group I and II than in group III, Occasionally there are several part of enchondral bone formation in the distraction site, especially in the external callus. Conclusively, preservation of the periosteum is much more important than preservation of the medullary cavity and endosteum in distraction osteogenesis. But further special study about the effect of TGF-beta should be followed.