Aim To observe the effect of oxidized low density lipoprotein (ox LDL) on the expression and activity of matrix metalloproteinases(MMPs). Methods Human monocyte derived macrophages were cultured in vitro and the gene and protein expressions of MMPs (MMP 2 and MMP 9) were detected by reverse transcription polymerase chain reaction and Western blot respectively. MMPs'activity was detected by Zymography. Results The results revealed that 10,20 and 40 g/L of ox LDL could stimulate the gene expression of MMP 2 and MMP 9 in comparison with the same concentrations of LDL. The expressions of MMP 2 protein and MMP 9 protein were higher in ox LDL groups than in LDL groups. The activity of MMP 2 and MMP 9 was also significantly higher in different concentrations of ox LDL groups than the same concentrations of LDL groups. The activity of MMP 2 and MMP 9 was in a dose dependent manner in ox LDL group. Conclusions The results indicate that ox LDL can obviously increase the expressions and activities of MMP 2 and MMP 9 in human monocyte derived macrophages. So ox LDL could enhance the breakdown of matrix in lesions of atherosclerosis which could probably induce vulnerable plaque. Rupture of the vulnerable plaque is the major cause of acute coronary events.