Urinary excretion of 8-iso-prostaglandin F-2 alpha (8-iso-PGF(2 alpha), a reliable biomarker for enhanced oxidant stress in vivo, has been described in association with diabetes and coronary heart disease. The aim of this study was to evaluate the relationship between urinary 8-iso-PGF(2 alpha) levels and the characteristics of coronary culprit lesion in diabetic patients with acute coronary syndrome (ACS). A total of 79 diabetic patients with ACS were included. iMAP intravascular ultrasound (iMAP-IVUS) was performed to evaluate the characteristics of culprit plaques. Fasting urinary 8-iso-PGF(2 alpha) level was measured and corrected by creatinine clearance. iMAP-NUS data showed culprit plaques in high urinary 8-iso-PGF(2 alpha) level patients had a greater percentage of necrotic core and less fibrous components. High urinary 8-iso-PGF(2 alpha) levels were correlated with increased necrotic plaque components (r = 0.325, P = 0.003). Meanwhile, the presence of thin-capped fibroatheroma (50.0% versus 11.5%, P = 0.003), ruptured plaques (30.8% versus 7.7%, P = 0.035), and thrombus (38.5% versus 7.7%, P = 0.008) were significantly more frequent in the upper tertile of urinary 8-iso-PGF(2 alpha) levels than in the low tertile. Multivariate analysis showed high levels of urinary 8-iso-PGF(2 alpha). (OR 4.240, P = 0.007) was independently associated with the presence of vulnerable culprit plaque in diabetic ACS patients. Urinary 8-iso-PGF(2 alpha) also displayed a significant value in predicting vulnerable plaques in diabetic patients with ACS by constructing the receiver-operating characteristic (ROC) curve (Area under the ROC curve: 0.713, P = 0.001). Urinary 8-iso-PGF(2 alpha) levels are associated with the vulnerability of the coronary culprit lesion in diabetic patients with ACS and may provide additional information for risk assessment in suspected vulnerable patients.