Objective To study the effect of continuous strain on the proliferation and osteogenic differentiation of rat bone marrow stromal cells(BMSCs) in vitro. Methods Rat BMSCs were obtained from adult female Sprague-Dawley rats (3-month old), and purified by full-blood attachment culture. BMSCs between passage 3—5 were seeded on Flexercell mechanical loading system(10%, 1 Hz), and divided into 1 h group, 6 h group, 12 h group, 24 h group, 48 h group, respectively, according to the time subjected to strain. Effects of continuous strain on the morphology, proliferation and osteogenic differentiation of BMSCs were observed and analyzed. Results (1) Compared with the control group, cells subjected to 10% strain showed the particular orientation. Their alignment elongated mostly in the direction perpendicular to the strain axis in a time-dependent manner. (2)10% continuous strain could significantly decrease the proliferation of BMSCs. (3) Continuous strain could increase mRNA expression of ALP, COLⅠand Runx2 in a time-dependent manner. Compared with the control group, mRNA expression of ALP was increased significantly at 24 h, COLⅠat 24 h and 48 h, and Runx2 at 6 h. mRNA expression of osteocalcin (OC) ascended greatly in the beginning, but went down gradually and was significantly lower than that of control at 48 h（P＜0.05）. (4) Continuous strain could induce an increase in Runx2 protein level. A sharp increase in Runx2 protein was observed at 6 h(P＜0.05) , then Runx2 protein level decreased slowly with its mRNA expression being significantly lower than that of control at 24 h（P＜0.05） Conclusions Continuous strain could induce rat BMSCs to orient in an orderly manner, suppress its proliferation activity, but stimulate the osteoblastic differentiation at the early stage.