Callus distraction had long been believed to be harmful to fracture healing until the development of a new method named callotasis, which gave the green light to the field of long bone lengthening. This revolutionary method showed that the distraction does not disturb bone formation, which was confirmed by our results of clinical cases. One of our callotasis cases was presented to show its characteristic X-ray features in the course of callotasis. We made experimental callotasis in rabbit femurs and performed mechanical tests for callus of the bones in order to investigate its mechanical behavior when tensile loads were applied. Femurs of eight rabbits were osteotomized at the level of the midshaft with a bone saw and fixed with a mini-model external fixator. Gradual callus distraction was performed at the rate of about 0.7 mm per day. The femurs were removed after sacrifice and served as bone specimens for the mechanical tests. Control long bones were twelve osteotomized femurs fixed with the same external fixator without callotasis and ten osteotomized tibiae fixed with an intramedullary Kirschner wire. This immature callus showed the characteristics of hysteresis in the load-deformation curves and also those of stress relaxation in stress relaxation test. We conclude that the callus in callotasis has the property of viscoelasticity, and infer that this property is one of the mechanical bases making callotasis possible.