In the study,the effect of sodium Tanshinone Ⅱ a sulfonate (TN Ⅱa) on c-myc expression in vascular smooth muscle (SMC) was explored with in situ hybridization. It was showed that macrophage-derived growth factor (MDGF) could apparently stimulate high gene expression of c-myc. In contrast, TN Ⅱ a could prevent MDGF’s stimulating effect on SMC, decrease the expression level of cmyc. The experiment showed TN Ⅱ a might play an important role on preventing the proliferation of SMC in atherosclerosis.