Bone grafts are used in the repair of segmental bone loss caused by severe trauma, bone tumors and infection, and to enhance bone healing in ununited fractures. Autograft is the most frequently used and the most effective method, but because of inadquate supply and additional operative morbidity, allograft or heterograft could be used. Heterograft has been shown to be poorly tolerated by the host and ineffective in providing an osteogenic system. The objective of this study is to observe healing process of a segmental defect of long bone following to heterograft, and to compare the difference of histologic process between autograft and hetero graft. Twenty-four white rabbits weighing 1,000 to 2,000 grams were used for the experiment. A segment measuring three times the diameter of the ulna shaft (1.5 to 2cm) of rabbit was resected with its periosteum. Twenty-four white rabbit were divided into three groups (control, autograft, heterograft group) according to graft methods, each group comprising of eight rabbits. The animals were sacrificed at 2, 4, 8 and 12 weeks after the experimental procedures and were periodically evaluated by radiographs and histology. The obtained results were as follows: 1. The results of the radiological evaluation showed that no ungrafted ulnar defects (control group) healed. In the heterograft group, partial union was observed from 8 weeks and complete union was obtained on 12 weeks. In the autograft group, partial union was observed on 2 weeks and complete union was obtained on 4 weeks. 2. The results from histological examination showed that the ungrafted ulnae did not heal across the defect but some immature trabeculae were founded on the edges of the defect. In the heterograft group, immature trabeculae were appeared from 4 weeks and defects were substituted with mature trabeculae on 12 weeks. In autograft group, defects were substituted with immature trabeculae on 2 weeks and with mature trabeculae on 4 weeks. As seen in the results of the experiment, union could be obtained with the heterograft, but needed more long time than autograft. Though the autograft is the most effective graft method in bone defect or non-union, above results suggest the heterograft as the alternative method in the treatment of large bone defect, in a multioperated patient, or in the children or the elderly, combined with autograft or alone.