BACKGROUND:: There is lack of study about bone marrow mesenchymal stem cells (BMSCs) and ultrastructure characteristics of osteoblasts after differentiation. OBJECTIVE:: To isolate and culture BMSCs by the whole bone marrow culture and adherence method, to induce the differentiation of BMSCs into osteoblasts, and to investigate the tiny and ultrastructure characteristics of induced and non-induced cells with an optical microscope and electron microscope. DESIGN, TIME AND SETTING:: This experiment was performed at the Department of Human Anatomy, Hebei Medical University and Department of Orthopaedics, Bethune International Peace Hospital from March 2007 to April 2008. MATERIALS:: Adult New Zealand rabbits were obtained from Animal Experiment Center of Bethune International Peace Hospital. Scanning electron microscope (SEM), Hitachi S-3500N and transmission electron microscope (TEM), Hitachi-7500 were used in this study. METHODS:: Bone marrow was collected from posterior superior iliac spine of New Zealand rabbits. After primary culture and passage, some the third passage of cells were incubated in 10-8mmol/L dexamethasone, 50 mg/L ascorbate and 10 mmol/L β -glycerol phosphate. BMSCs were identified by alkaline phosphatase (ALP), bone morphogenetic protein and calcified node staining. MAIN OUTCOME MEASURES:: General morphological characteristics of BMSCs and differentiated osteoblasts were observed by the optical microscope and their ultrastructural characteristics by the electron microscope. RESULTS:: At the third passage, BMSCs were positive for ALP staining and negative for bone morphgenetic protein (BMP) by immunohistochemical staining and calcified node staining, but induced BMSCs were positive for ALP staining, BMP immunohistochemistry and calcified node stainings. The BMSCs showed long fusiform shape mostly and a few showed astero-form by the optical microscope and scanning electron microscope. After induced, cell volume increased, the abundant organellas in BMSCs were observed by transmission electron microscope. After induced, mitochondrion increased and a lot of matrix vesicles, myelinated-like body and polyvacuolus-like structure appeared in cytoplasm. CONCLUSION:: Increased matrix vesicles, myelinated-like body and polyvacuolus-like structure in cytoplasm maybe the ultrastructural characteristics of BMSCs after induced differentiating into osteoblasts.Wang ZS, Wang JY, Wang BZ, Wang L, Liu CA, Wang L, Liu GS. Ultrastructure characteristics of osteoblasts differentiated from bone marrow mesenchymal stem cells. Zhongguo Zuzhi Gongcheng Yanjiu yu Linchuang Kangfu 2009;13(1):11-16 [http://http://www.crter.cn http://en.zglckf.com]