Aim To observe serum PON-1 activity in patient with acute coronary syndrome(ACS)and investigate its role in acute coronary syndrome. Methods Serum PON-1 activity was assessed by use of phenylacetate(arylesterase) as substrate. PON-1 activity of 40 patients with acute coronary syndrome, 32 patients with stable coronary heart disease(SCHD) and 24 controls were determined by the use of a recording spectrophotometer. Results In patients with acute myocardial infarctioon(AMI) and unstable agina pectoris(UAP),serum PON-1 activity[95±42 μmol/(L·min), 80±36 μmol/(L·min), respectively] were significantly lower(p<0.05) than those in patients with SCHD [133±29 μmol/(L·min)] and controls [136±64 μmol/(L·min)];there was no difference between SCHD group and controls. Serum PON-1 activity was positively correlated with serum triglyceride levels(r=0.496, P=0.001) and with ApoB(r=0.342, P=0.002),but serum PON-1 activity was not correlated with high density lipoprotein cholesterol(HDLC). Conclusions The low serum PON-1 activity in patients with ACS indicated the reduction of antioxidant properties may involve in the production of ACS.