In this study, the investigates try to confirm our hypothesis that low dose ticagrelor(60mg) had similar anti-platelets function compared with the standard dose ticagrelor in senior （no less than 75 years old)acute coronary syndrome patients . Totally 40 senior ACS patients will be divided into 2 groups randomly one month after PCI . Group 1 will be treated with ticagrelor 90mg and aspirin 100mg after PCI for 12 months as the standard group; Group 2 will be treated with ticagrelor 60mg plus aspirin 100 mg for 11 months after one month standard DAPT treatment. The platelets function will be tested in VASP and TEG methods 2 months after PCI as the primary endpoints.The clinical events will be observed 12 months after PCI.

Inclusion Criteria:

1.Subject must be at least 75 years of age 2 Patients should have undergone successful percutaneous coronary intervention with drug-eluting stent for acute coronary syndrome 3.Subject understand the study requirements and the treatment procedures and provided informed consent before the procedure

Exclusion Criteria:

1. Female of childbearing potential, unless a recent pregnancy test is negative, who possibly plan to become pregnant any time after enrollment into this study
2. Active bleeding
3. Known hypersensitivity or contraindication to study medications
4. Female of childbearing potential, unless a recent pregnancy test is negative, who possibly plan to become pregnant any time after enrollment into this study
5. Non-cardiac co-morbid conditions are present with life expectancy <2 year or that may result in protocol non-compliance (per site investigator's medical judgment).
6. Subjects with Cerebral hemorrhage history
7. Subjects with stroke history in half a year
8. subjects with active malignant tumor
9. subjects with whom oral anticoagulants are needed
10. Other conditions which the investigators think not applicable to the study.