Objective To observe platelet aggregation and changes of platelet activation indexes including P-selection and membrane GP IIb/IIIa in patients with acute coronary syndromes (ACS) and to explore the role of activated platelets in the pathogenesis of ACS. Methods 52 healthy people were selected as the normal control group, while 89 patients with ACS were selected and divided into two observation groups: UA group (unstable angina, n=43) and AMI group (acute myocardial infarction, n=46). The platelet aggregation rate and activation indexes expressed on the surface of platelet such as p-selection and membrane GP IIb/IIIa were detected in patients with ACS. Results Compared to the normal control group, the rate of platelet aggregation in UA group was obviously enhanced (P<0.001); the platelet p-selection and membrane GP IIb/IIIa were also significantly increased with significant differences (P<0.001). For ACS patients, compared to UA group, patients of AMI group had significantly higher platelet aggregation rate (P<0.001); and obviously higher platelet p-selection and membrane GP IIb/IIIa (P<0.001). Conclusion  As the aggregation of disease in patients with ACS, the platelet activation level is increased greatly and the aggregation function is enhanced obviously. The abnormal changes of platelet functions in patients with ACS are closely related to the level of diseases.