Objective: To study the differentiation of bone marrow mesenchymal stem cells (BM-MSCs). When co-cuttured with tooth germ cells (TGCs). Methods: The BM-MSCs were isolated from tibiae and fibula of Sprague-Dawley (SD) rats aseptically and cultured. The fist molar tooth buds were isolated from E17 SD foetus and processed to obtain TGCs. The BM-MSCs were co-cultured with TGCs indirectly. The differentiation potentiality of BM-MSCs and the tooth-forming potentiality of TGCs were assessed. Results: The BM-MSCs started to express odontogenic markers, dentin sialoprotein (DSP) and dentin matrix protein 1 (DMP1) after 7 days co-culture with TGCs. Implants of co-cultured pellets formed osteoid dentin under renal capsules, while control groups self-differentiated into osseous tissue. Conclusion: BM-MSCs could be differentiated by co-cultured with TGCs into dental tissues, although the molecular mechanism requires further study. Keywords: co-culture, differentiation, mesenchymal stem cell, tooth germ