The migration and matrix metalloproteinase (MMP) activation of vascular smooth muscle cells may play key roles in the development of atherosclerosis. Carnosic acid (CA) is a phenolic compound found in herbs, including rosemary and sage. Previous studies indicated that CA possesses antioxidant activity in vitro. In this study, we investigated the effects of CA on TNF-α-induced cell migration, the formation of intracellular reactive oxygen species, the translocation of NF-κB and the activation and expression of MMP-9 in human aortic smooth muscle cells (HASMC). The Matrigel migration assay showed that CA (10 and 20 μmol/l) effectively inhibited TNF-α-induced migration of HASMC as compared with the control group. To explain this inhibitory effect, MMP-9 was assayed by gelatin zymography and Western blot. The results indicated that CA inhibited MMP-9 activity and expression. Furthermore, the production of reactive oxygen species and the nuclear translocation of NF-κB p50 and p65 induced by TNF-α were dose-dependently suppressed by CA pretreament. These results indicate that CA has anti-inflammatory properties and may prevent the migration of HASMC by suppressing MMP-9 expression through down-regulation of NF-κB.