In order to define the changes of mechanical properties of bone tissue in the growing period, 90 rabbits' tibiae were loaded in 3-point bending. Emphasis was given to the accurate measurement of maximum angulation degrees within the limit of elastic deformation. Young's modulus of elasticity increased with increasing body weight, and the increments of bending stiffness were more prominent. In rabbits of low body weights, the tibiae absorbed more energy before fracture occurred and plastic deformation after fracture occurred more frequently. The mean values and standard deviations of the maximum angulation were 9.84±1.33 degree in Group 1 (range of body weight: 800–1200g), 7.55±0.85 degrees in Group 2 (range of BW: 1600–2000g), and 5.90±0.78 degrees in Group 3 (range of BW: 2400–2800g), These data may support that the allowable maximum angular deformity in treatment of fracture should be adjusted according to the increase of body weight-in other words, maturation.