Silk fibroin has been widely tested as a candidate biomaterial applicable to various attempts of tissue engineering. In order to examine bone forming ability of silk 3-D scaffold, we have developed a tibial interlocking intramedullary nailing model. A tibial intramedullary nail in the weight bearing hind limb of a rat was interlocked with a pin through a pin hole at the proximal end of the intramedullary nail. Interlocking of the intramedullary nail prevented total collapse of proximal region of a bone defect and helped maintain the critical gap that was filled with silk 3-D scaffold, though minor shinkage about 1 mm at distal region was unavoidable. Bone forming ability of an implanted silk scaffold was monitored weekly for 8~10 wks by X-ray radiography of live animals and bone formation in the scaffold was examined by H & E staining and Masson’s trichrome staining of the bone tissue recovered from the animals. Although scattered islets of bone tissue was observed in the implanted silk scaffold, bone tissue was not widely developed and implanted scaffolds of silk nanofiber and salt-leached sponge were X-ray transparent, suggesting inefficient bone formation. By contrast, X-ray image of implanted silk nanofibrous scaffold coated with hydroxyapatite was getting darkened with time, which suggests bone tissue formation in the scaffold, while untreated silk 3-D scaffold remained undisturbed. Although existence of bone tissue in the scaffold should be confirmed by histological criteria, hydroxyapatite-coated silk scaffold appeared competent to support regeneration of the long bone defects In addition, the interlocking intramedullary nailing in tibia of rat hind limb could be applicable in assessing long bone regenerative capacity of various biocompatible materials.