Objective To investigate the bioactivity of osteocytes in tiny morselized bone grafts for bone defect repair. Methods Models of radial defect were established on 60 female svngeneic inbred DA rats which were divided into 3 groups randomly:structural bone grafting group(n=56),tiny morselized bone grafting group(n=56)and control group(without bone grafting, n=4).The ilia of 28 male inbred DA rats were harvested as donors and made into structural bone grafts 2mm in diameter and tiny morselized bone grafts 300～500 μm in diameter to be transplanted into the radial defects of the female receptors.Samples were harvested on 1 d,4 d, 1 w,2 w,4 w,6 w and 10 w after transplantation.The presence and relative amounts of genes specific to the sex-determining region of the Y-chromosome(Sty)originating from the bone grafts were measured by in situ hybridization(ISH).The expressions of bone morphogenetic proteins-2(BMP-2),transforming growth factor-betal (TGF-β1),alkaline phosphatase (ALP)and Collagen I were semiquantmed by immunohistochemistry. Results In structural bone grafting group,the expression of Sry decreased early and disappeared at 1 w.But after 4 w,Sry appeared again and its expression increased with lapse of time.In tiny morselized bone grafting group,Sry was detected all the time.At each time point.its expression was more than in Group I(P＜0.05).Donor bone cells were different during repair in both groups.Tiny morselized bone grafts were absorbed quickly,and the expressions of BMP-2 and TGF-β1 reached the peak faster than in structural bone grafting group.The positive cells of BMP-2,TGF-β1,ALP and Collagen I in tiny morselized bone grafting group were much more than in structural bone grafting group during the first 2 w(P＜0.05).Conclusions Both bone grafts can provide donor cells to repair bone defects.Compared with bulk bone grafts,tiny morselized bone grafts contain more living bioactive osteocytes which can secrete bone growth factors and bone matrix proteins,so they may accelerate healing of bone defects.