In order to investigate wheter or not elastic dynamization by axial micromotion enhanced regenerate bone healing in distraction osteogenesis(callotasis), a unilateral external fixator, with which the magnitude of dynamization could be controlled, was applied to the tibiae of 12 Korean adult mongrel dogs, weighting approximately 35kg. Tibiae and fibulae of both sides were osteotomized at the proximal metaphysio-diaphyseal junction and fixed with unilateral external fixators. Distraction was started from the third postoperative day until 2 cm (10% of tibial length) of length gian was achived, at the rate of 0. mm twice a day. When 1 cm (5%) of lengthening was achieved, controlled elastic dynamization of 1 mm (group I), 2 mm (group II), and 3 mm (group III) in magnitude was allowed on left tibia. Right tibia was not dynamized until sacrifice, and served as a control for each group. Plain anteroposterior radiographs and measurements of bone mineral density(BMD) by dual x-ray absorptiometry(DXA) were obtained pre- and post-operatively. Relative ratio of BMD was calculated, dividing the BMD value measured in each period by the BMD value measured at the beginning of dynamization. Mechanical test was performed in uniaxial compression on an Instron machine. Following observations were made: 1. Plain anteroposterior radiography showed that at mid-consolidation phase (postoperative 50th day) the width of callus in the distraction gap increased significantly in all dynamization groups, as compared to their control groups(p<0.05). However, there were no significant differences among the dynamization groups. 2. Both BMD and the relative BMD ratio of the distraction gap were higher in the dynamization groups than those in the their control groups throughout the study period(p<0.05). However, there was no significant difference among the dynamizaion subgroup(p>0.05). 3. Axial Compression test revealed that the dynamization group had significantly increased stiffness, maximal load, elastic modulus, and strength, as compared to those of the control group(p<0.05). In conclusion, controlled elastic dynamization effectively enhances regenerate bone healing in distraction osteogenesis of canine tibia, at 1 mm to 3 mm of magnitude.