We hypothesized that ticagrelor monotherapy might be enough to prevent thromboembolic events without aspirin after PCI in patients with acute coronary syndrome(ACS). Moreover, ticagrelor monotherapy will reduce bleeding risk compared to DAPT with aspirin plus ticagrelor. We will also evaluate 1-year safety and efficacy of Orsiro stent for patient with acute coronary syndrome. After confirmation of enrollment, patients will be randomized to continue standard treatment (aspirin plus ticagrelor) for 1 year or to stop aspirin after discharge or less than 1 month after PCI (ticagrelor monotherapy). Randomization will be stratified according to 1) the presence of diabetes and 2) ST elevation myocardial infarction (MI). Baseline clinical and angiographic characteristics, laboratory findings will be assessed at the time of randomization. All patients will provide informed consent on their own initiative.

Inclusion Criteria:

1. Patients ≥19 years old
2. Patients who received new generation sirolimus-eluting (Orsiro® series) stent implantation for treating ACS, including acute MI and unstable angina
3. Provision of informed consent

Exclusion Criteria:

1. Age> 80 years
2. Increased risk of bleeding, anemia, thrombocytopenia
3. A need for oral anticoagulation therapy
4. Pregnant women or women with potential childbearing
5. Life expectancy < 1 year