Objective To investigate the regulation of proliferation of rat marrow mesenchymal stem cells (rMSCs) by mechanical stretch. Methods rMSCs were isolated and purified by density gradient centrifugation, expression of relative surface antigens was detected by immunochemistry staining, and distribution of cell cycle was analyzed with flow cytometry. rMSCs were exposed to stretch with different time and magnitude, MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrasodium bromide) was used for proliferation assay and expression of c-fos gene was examined by RTPCR. Results rMSCs could be perfectly achieved by the combination of density gradient centrifugation and adhesion difference, and cultured stably in DMEM medium containing 15% fetal bovine serum (FBS). The cells exhibited an elongated fusiform shape and CD44 and Fibronectin positive while CD34 negative. Flow cytometry revealed that G0/G1 phase cells amount to 90%. MTT assay showed that O.D values of rMSCs increased after exposure to stretch, and revealed a time-and strain-dependence manner within all time and stretch ranges studied. In contrast to control group, expression of c-fos gene in rMSCs showed a significantly increase after exposure to 1Hz, 8% strain for 60 minutes. Conclusion mechanical stimulation is crucial to regulating the cell growth and proliferation. The appropriate mechanical stretch could be an accelerant for proliferating capacity of rMSCs in vitro.