Aim To investigate the underlying molecular mechanism of probucol stabilizing plaque and preventing acute coronary syndromes. Methods Reverse transcription polymerase chain reaction (RT PCR), Western blot and gelatin zymography were performed to detect matrix metalloproteinase 9 (MMP 9) expression,secretion and activity in THP 1 monocyte derived macrophages induced by oxidized low density lipoprotein (ox LDL). Results The expression and enzyme activity of MMP 9 significantly increased in THP 1 monocyte derived macrophages exposed to ox LDL (80 mg/L) for 12 h. Probucol at a concentration of 20, 40 and 60 μmol/L inhibited MMP 9 protein secretion by 27.0%±2.6%,45.0%±5.5% and 74.0%± 2.4% respectively (p<0.05), and decreased enzyme activities by 29.0%±3.2%,34.0%±1.9% and 48.0%±5.1% respectively (p<0.05), but had no influence on MMP 9 mRNA expression. Conclusion Probucol can effectively inhibit MMP 9 secretion in macrophages, which may play an important role in stabilizing plaque and anti atherosclerosis.