We investigated the effects of resveratrol on metastasis in in vitro and in vivo systems. 4T1 cells were cultured in the presence of various concentrations (0-30 µmol/L) of resveratrol. For experimental metastasis, BALB/c mice were injected intravenously with 4T1 cells in the tail vein, and were orally administered various concentrations (0, 100, or 200 mg/kg Body weight) of resveratrol for 21 days. After resveratrol treatment, cell adhesion, wound migration, invasion, and MMP-9 activity were significantly decreased in a dose-dependent manner in 4T1 cells ( P < 0.05). The numbers of pulmonary nodules were significantly decreased in mice fed the resveratrol ( P < 0.05). The plasma MMP-9 activity was decreased in response to treatment with resveratrol in mice ( P < 0.05). We conclude that resveratrol inhibits cancer metastasis both in vitro and in vivo , and this inhibition is likely due to the decrease in MMP-9 activity caused by resveratrol.