Objective To study the values of standard 12-lead electrocardiogram (ECG) in differential diagnosis of acute pulmonary embolism (APE) and non-ST elevation myocardial infarction (NSTEMI). MethodsA retrospective analysis was conducted on 126 patients who were treated in the First Affiliated Hospital of Wenzhou Medical College during Jan. 2005 to Jan. 2011. The patients included 42 patients with APE (mean age [61±12] year) and 84 with NSTEMI (mean age [72±15] year). The data of all patients were complete and true, and the two groups were comparable in age and sex distribution. The standard 12-lead ECG records on admission were analyzed and the parameters which could be used for differential diagnosis were screened. Results Frequencies of right bundle branch block (RBBB) and SⅠQⅢTⅢ or SⅠSⅡSⅢ pattern were similar in the two groups ([11.9%] APE vs [14.3%] NSTEMI, [26.2%] APE vs [15.5%] NSTEMI patients, respectively). Negative T waves in leads Ⅴ1-Ⅴ3 together with negative T waves in inferior wall leads Ⅱ, Ⅲ, aVF (OR 1.32, 95%CI[1.15-1.69]) yielded a positive predictive value of 88% and specificity of 82% for APE. However, ST depression in leads Ⅴ5-Ⅴ6 and negative T waves in leads Ⅴ5-Ⅴ6 (OR 1.85, 95%CI [1.14-3.01]) yielded a positive predictive value of 89% and specificity of 50% for NSTEMI. ConclusionRBBB and SⅠQⅢTⅢ or SⅠSⅡSⅢ pattern may not help to differentiate between APE and NSTEMI. Coexistence of negative T waves in precordial leads Ⅴ1-Ⅴ3 and inferior wall leads Ⅱ, Ⅲ, aVF may suggest APE diagnosis. Coexistence of negative T waves and ST segment depression in precordial leads Ⅴ5-Ⅴ6 may suggest NSTEMI diagnosis.