PURPOSE: The purpose of this study was to evaluate the bone regeneration capacity of silk fibroin (SF) when combined with beta tricalcium phosphate (beta-tricalcium phosphate [TCP]) and rh-bone morphogenetic protein (BMP) in vivo by micro-computed tomography (CT), soft x-ray, and histological analysis. METHODS: A total of 56 critical size defects formed by a trephine bur made on 28 adult female Spague-Dawley rats were used for this study and the defect size was 5.0 mm in diameter. The defects were transplanted with (1) no graft material (raw defect), (2) autogenous bone, (3) SF (10 microg), (4) SF-BMP (10 microg, 0.8 microg each), and (5) SF+beta-TCP (10 microg). At 4 and 8 weeks after operation, the experimental animals were sacrificed. Samples were evaluated with soft x-ray, histological examinations and 3-dimensional micro-CT analysis. RESULTS: In the 3-dimensional micro-CT evaluation, bone volume and bone surface data were higher in the SF-BMP (12.8+/-1.5, 138.6+/-45.0 each) (P<0.05) and SF-TCP (12.3+/-1.5, 144.9+/-30.9 each) group than in the SF group (6.1+/-3.3, 77.2+/-37.3 each) (P<0.05), except for the autogenous group (15.0+/-3.0, 190.7+/-41.4 each) at 4 weeks. At 8 weeks, SF-BMP (16.8+/-3.5, 173.9+/-34.2 each) still revealed higher (P<0.05) bone volum and surface, but SF-TCP (11.3+/-1.5, 1132.9+/-52.1 each) (P=0.5, P=0.2) revealed the same or lower amount compared with the SF group (13.8+/-2.7, 127.5+/-44.8 each). The % of bone area determined by radiodensity was higher in the SF-TCP (31.4+/-9.1%) and SF-BMP (36.2+/-16.2%) groups than in the SF (19.0+/-10.4) group at the period of 4 weeks. Also, in the histological evaluation, the SF-BMP group revealed lower inflammation reaction, lower foreign body reaction and higher bone healing than the SF group at postoperative 4 weeks and 8 weeks. The SF-TCP group revealed lower inflammation at 4 weeks, but accordingly, as the TCP membrane was absorbed, inflammatory and foreign body reaction are increased at 8 weeks. CONCLUSION: The current study provides evidence that the silk fibrin can be used as an effective grafted material for tissue engineering bone generation through a combination of growth factor or surface treatment.