Abstract：Objective：To investigate the level of plasma D dimer (D-D) and its clinical application in the patients with acute coronary syndrome (ACS). Methods：The level of plasma D-D were measured and compared in 75 healthy controls, 149 patients with unstable angina pectoris (UAP), 101 patients with non-ST-segment elevation myocardial infarction (NSTEMI) and 107 patients with ST-segment elevation myocardial infarction (STEMI). The diagnostic value of D-D for ACS was assessed by ROC curve. Results：The median (P₂₅，P₇₅) of plasma D-D level in healthy controls and the patients with UAP, NSTEMI and STEMI were 329 (126, 537), 634 (200, 1 333), 771 (299, 1 400) and 1 160 (388, 2 418) μg/L respectively, and those in the patients with single vessel lesion, double vessel lesion and triple vessel lesion were 817 (200, 1 751), 1002 (262, 2 418) and 1 276 (322, 2 341) μg/L respectively. The level of plasma D-D in the patients with STEMI was higher than that in the patients with UAP and NSTEMI (U=198.1, 96.5, P<0.01). The level of plasma D-D in the patients with triple vessel lesion was higher than that in the patients with single vessel lesion (U=459.9,P<0.01). The area under ROC curve (AUC^ROC) of D-D for STEMI diagnosis was 0.866(95%CI, 0.793-0.939). When the cut off value was 793 μg/L for STEMI diagnosis, the sensitivity and specificity were 84.8% and 69.3% respectively. The Cox model multi-analysis showed that high level plasma D-D was the independent mortality risk predictor in ACS patients. Conclusion: The level of D-D showed high sensitivity for diagnosis of STEMI, so it should be a useful marker for estimating prognosis of the patients with STEMI.