Aim To investigate the effect of metformin on intimal hyperplasia after arterial injury and explore the underlying mechanisms. Methods Male SD rats were randomly divided into control group, carotid arterial balloon injury group and carotid arterial balloon injury plus metformin group, 15 rats in each group. After the intervention for three weeks, the histological structure of carotid artery was evaluated by HE staining, the expressions of matrix metalloproteinase （MMP）-2 and MMP-9 were determined by real-time PCR, Western blot and immunohistochemistry staining. Results Intimal hyperplasia was caused by carotid arterial balloon injury. The expressions of MMP-2 and MMP-9 in carotid artery were significantly upregulated after balloon injury （P<0.01）. Treatment of metformin attenuated the upregulation of MMP-2 and MMP-9 and inhibited intimal hyperplasia induced by arterial injury （P<0.05 or P<0.01）. Conclusions Metformin may prevent intimal hyperplasia through inhibiting MMP-2 and MMP-9 expression after arterial injury.