It is essential to define the mechanical and morphological characteristics of bone in undrestanding the mechanism and treatment of fractrues of bone. We already reported the results of bending study in 1983 and tension study in 1987 to assess the mechanical and morphological changes of bone according to the growth. Following these studies, we performed torsion study on growing long bone to have better understanding of the characteristics of bone. In order to assess the mechanical and morphological properties of fracture of growing bone by torsion, we divided 100 Newzealand white rabbits(40 1-month-old rabbits, 20 3-months old rabbits and 40 5-months-old rabbits) into 6 experimental groups according to their maturity and side : Group 1(right femur of 1 month old), Group 2( left femur of 1 month old), Group 3(right femur of 3 months old), Group 4(left femur of 3 months old), Group 5(right femur of 5 months old), Group 6(left femur of 5 months old). Each group of femora were loaded in torsion with varing distance and the following results were obtained: 1. Fracture shapes were composed of the spiral component and the vertical component which connected the two ends of the spiral component. 2. Younger bones revealed greater deformation angle and apex angle but less obliquity than older bones. 3. When the gap between the grips were lengthened three times, there was a very signi ficant increase of deformation angle and obliquity but apex angle was decreased and the fracture angle showed little change. 4. It was necessary to measure the apex angle and obliquity in the morphological description of fractures by torsion. 5. We observed when a certain degree of angular deformity was applied on a bone at a certain age, the fracture was occured through the tension failure regardless of types of loading forces.