Bone is a heterogenous and anisotrophic material exhibiting elastic, viscoelastis and plasticproperties. A considerable amount of information is now available regarding biomechanical properties and mechanisms of fractures of mature long bones, but less is known about the properties of the growing bones. In order to assess the biomechanical properties under tension, rabbits growing bones were loaded with Instron Modal 1,000 until fracture occurred. Two hundred tibiae of 100 New Zealand white immature rabbits were divided into 6 experimental groups according to maturity and size. Group I-R(40 right tibiae of 1-month old rabbits), Group I-L (40 left tibiae of 1-month old rabbits), Group II-R(20 right tibiae of 3-month old rabbits), Group II-L (20 left tibiae of 3-month old rabbits), Group III-R(40 rigth tibiae of 5-month old rabbits), and Group III-L(40 left tibiae of 5-month old rabbits). The following results were obtained. 1. Fracture type was transverse.or nearly transverse in 170 out of the total 200 tibiae(85%). 2. Maximum tensile strength of each group was 628.4+170.1N(Group I -R), 619.4+177.5N(Group I-L), 1,111.0+307.4N(Group II-R), 1,147.2+361.6N(Group II-L), 1,590.3+379.9N(Group III-R), 1,696.6+465.4N(Group III-L). 3. Ultimate tensile stress of each group was 87.0+24.8 MN/m(Group I-R), 96.4+25.6MN/m(Group I-L), 126.3+31.6MN/m(Group II-R), 132.0+44.2MN/m(Group II-L), 139.9+37.5MN/m(Group IU- R), and 143.0+37.9MN/m(Group III-L). 4. Ultimate tensile strain of each group was 0.12+0.04(Group I-R), 0.11+0.07(Group II-L), 0.10+ 0.04(Group III-R), and 0.10+0.05(Group III-L). 5. Young's modulus of each group was 804.2+373.9MN/m(Group I-R), 881.9+396.3MN/m(Group I-L), 1,474.8+387.4MN/m(Group II R), 1,450.7+561.3MN/m(Group II-L), 1,642.6+ 651.9MN/m (Group III-R), and 1,772.0+713.3MN/m(Group III-L). 6. Energy absorption capacity of each group was 5.42+2.70MN/m(Group I-R), 6.00+2.69MN/m (Group I- ), 6.54+3.66MN/m(Group II-R), 6.58+3.78MN/m(Group II-L), 7.48+4.13MN/m(Gro-III-R), and 8.43+3.96MN/m(Group III-L). Based on this experiment, following conclusions were mabe. 1. Fractures were usually transverse in type. 2. Younger bones showed lower maximum tensile stregth than older bones. 3. Younger bones showed lower ultimate tensile stress and lower Young's modulus than older bones. There was statistically significant difference between 1- and 3-month old rabbits but not between 3- and 5-month old rabbits. 4. Younger bones showed lower energy absorption capacfty than older bones, but there was statistically significant difference only between 1- and 5-month old rabbits. 5. Younger bones showed higher ultimate tensile strain than older bones, but there was statitically significant difference only between 1- and 5-month old rabbits. 6. Between right and left bones, all biomechanical properties showed no statistically significant differences.