Abstract: Objective T o observe the role of paraoxonase-1 and oxidative stress in pathogenesis of insulin resistance in patients with polycystic ovary syndrome (PCOS). Methods 41 PCOS patients divided into obese group and non-obese group with 20 healthy volunteers as a control. The 75g oral glucose tolerance test (OGTT) and insulin releasing test (IRT) were performed. The insulin resistance and β-cell function were estimated with homeostasis model assessment (HOMA). The fasting serum concentration of malondialdehyde (MDA), and the activities of superoxide dismutase (SOD) and paraoxonase-1 (PON-1) were assayed. Results (1) The levels of fasting plasma insulin, 2 hour plasma glucose and insulin, the index of insulin resistance (HOMA-IR) and β-cell function (HOMA-β) were significantly higher in PCOS patients of both non-obese group and obese group than in healthy controls(all P<0.05). HOMA-β was not different between non-obese group and obese group in PCOS patients(P>0.05), while the the levels of BMI, 2hPG, FIns, 2hIns, and HOMA-IR were significantly higher in obese group than in non-obese group(all P<0.05). (2)The PCOS patients in both non-obese group and obese group showed higher MDA and lower SOD and PON-1 than in healthy controls(P<0.05). The serum concentration of MDA and the activity PON-1 showed no differences between non-obese group and obese group in PCOS patients (both, P>0.05), but The activity of SOD was significantly lower in obese group than in non-obese group(P<0.01). (3) SOD had a positive correlation with PON-1(r=0.417, P=0.043), and a negative correlation with HOMA IR(r=-0.492, P=0.015)in non-obese PCOS group. Conclusion The PCOS patients has an increased postprandial plasma glucose, impared PON-1 activity, oxidative stress, insulin resistance, and high level of plasma insulin in non-obese and especially in obese subgroup. Insulin resistance is related to oxidative stress in patients with PCOS.