Background: This study aims to measure platelet aggregation with thromboelastography (TEG) and observe low response to clopidogrel in patients with acute coronary syndrome (ACS) receiving dual antiplatelet therapy. Methods: TEG was used to measure platelet aggregation in 167 hospitalized patients with ACS. Low response to clopidogrel and aspirin refer to adenosine diphosphate (ADP)-induced platelet aggregation >= 70% and arachidonic acid (AA)-induced platelet aggregation>50%, respectively. Results: Low response to clopidogrel was observed in 50 patients (29.9%) and 16 patients (9.6%) showed poor response to both clopidogrel and aspirin. The differences in gender, smoking history and total cholesterol (TC) on admission were statistically significant between patients with low and normal response to Clopidogrel (P < .05). The multivariate Logistic regression analysis showed that low response to aspirin and daily smoking 10 or less cigarettes were the risk factors of low response to Clopidogrel (the odds ratios were 1.047, P = .000 and 2.987, P = .007). Conclusion: Partial ACS patients receiving standard antiplatelet therapy exhibited low response to clopidogrel, which was not affected by the age, combined therapy, or administration method of Clopidogrel, and the patients with low response to aspirin and daily smoking 10 or less cigarettes were more susceptible to low response to clopidogrel. Copyright (c) 2018, Taiwan Society of Geriatric Emergency & Critical Care Medicine. Published by Elsevier Taiwan LLC.