BACKGROUND:: The repair of bone defect in osteonecrosis of femoral head is difficult, and the ultrastructure of the osteogenic process porous bioceramic β-tricalcium phosphate (β-TCP) combined with bone marrow mesenchymal stem cells (BMSCs) in vivo still remains unclear. OBJECTIVE:: To observe the ultrastructure of the repair of porous bioceramic β-TCP combined with BMSCs in osteonecrosis of femoral head model, to test the osteogenesis during the repairing process, and to verify the feasibility that porous bioceramic β-TCP is an excellent scaffold of tissue engineering. DESIGN, TIME AND SETTING:: A randomized controlled research on animals was performed in Bone Circulation and Osteonecrosis Laboratory of China-Japan Friendship Hospital from February to August in 2008. MATERIALS:: Rabbit BMSCs were cultured in vitro, and then combined with the porous bioceramic β-TCP for 2 weeks. METHODS:: Thirty New Zealand white rabbits were divided into three groups. Subsequent to the establishment of osteonecrosis of femoral head model, blank control group received no padding, β-TCP group was transplanted with β-TCP, and β-TCP + BMSCs group was transplanted with the compound of the two. MAIN OUTCOME MEASURES:: The compound of β-TCP with BMSCs was observed under environmental scanning electron microscope; The specimens were collected postoperative 6 and 12 weeks, and the ultrastructure in the repair of β-TCP combined with BMSCs was observed by electron microscope and the substitution of osteogenesis and material degradation was also investigated. RESULTS:: The BMSCs could adhere, grow, proliferate and differentiate in porous bioceramic β-TCP. Both 6 and 12 weeks postoperatively, fibrous tissue was still found in the defect area of blank control group whereas no bone trabecula was observed. In the β-TCP group, the material began to degrade and not clear with matrix 6 weeks postoperatively, the material was identical with periphery area 12 weeks postoperatively and porous frame structure began to disappear. In the β-TCP + BMSCs group, new bone formed at 6 postoperative weeks, the parallel trabecularism formed and the degradation was obvious. A lot of osteoblasts were found in the periphery area. Compared with blank control group, the repairing and osteogenesis effect was better in other two groups, especially in the β-TCP + BMSCs group. CONCLUSION:: Porous bioceramic β-TCP is a fine scaffold of tissue engineering, easy to combine with BMSCs, and suitable for repairing of bone defect in osteonecrosis of femoral head.Sun W, Li ZR, Wang BL, Wu JQ, Shi ZC, Shi SH, Liu BL, Zhao FC. Porous bioceramic beta-tricalcium phosphate combined with bone marrow mesenchymal stem cells in the repair of osteonecrosis of the femoral head model: An ultrastructure observation.Zhongguo Zuzhi Gongcheng Yanjiu yu Linchuang Kangfu 2009;13(3):457-460 [http://www.crter.cn http://en.zglckf.com]/