1. Ticagrelor inhibits inflammation and improves vascular endothelial cell function to a greater extent than clopidogrel in ST-segment elevation myocardial infarction(STEMI) patients receiving percutaneous coronary intervention.
2. Ticagrelor can reduce the serum levels of inflammatory biomarkers both in coronary and in peripheral venous in patients with ST-segment elevation myocardial infarction(STEMI).

Platelet participates in the process of forming and extending atherosclerotic plaques, and it is also a source of inflammatory mediators. This study is a randomized, open-label study, designed to test the hypothesis that ticagrelor inhibits inflammation and improves vascular endothelial cell function to a greater extent than clopidogrel in ST-segment elevation myocardial infarction(STEMI) patients receiving percutaneous coronary intervention. Patients who are scheduled to undergo emergency Percutaneous Coronary Intervention(PCI) will be randomly assigned to receive ticagrelor 180mg for treatment group or clopidogrel 600mg for control group ,then after Percutaneous Coronary Intervention(PCI) treatment group treated with ticagrelor 90mg twice daily while the control group received clopidogrel 75mg once a day . All patients should receive Acetylsalicylic Acid(ASA) 300 mg as a loading dose before Percutaneous Coronary Intervention(PCI) then 100 mg daily unless intolerant. Glycoprotein Ⅱb/Ⅲa receptor antagonists and low-molecular-weight heparin and other additional medication will be directed by the treating cardiologist. All interventions will be performed via the radial approach with the standard technique within 12h after they are involved, and drug-eluting stents will be placed according to stenosis of coronary artery.

The vascular endothelial function will be tested by Circulating Endothelial Cells (CECs) and levels of inflammation will be tested by CD40 ligand (CD40L), C-reactive protein (CRP), and P-selectin to identify that ticagrelor inhibits inflammation and improves vascular endothelial cell function to a greater extent than clopidogrel.

Inclusion Criteria:

1. Male or non-pregnant female.
2. Age ≥ 18 years old and <80 years old.
3. Consecutive patients who should be hospitalized with documented evidence of ST-Segment Elevation Myocardial Infarction receiving Percutaneous Coronary Intervention.
4. All patients havepersistent≥0.2 Millivolt ST segment elevation in two or more contiguous precordial leads or ≥0.1 Millivolt ST elevation in two or more contiguous limb leads, with one of the following: persistent chest pain or elevatory of biomarkers of myocardial necrosis.
5. Time from chest pain onset to receiving Percutaneous Coronary Intervention <12 hours.
6. Persistent chest pain <12 hours.
7. Provision of informed consent prior to any study specific procedures.

Exclusion Criteria:

1. Involved in other trials.
2. In recent one year have P 2 Y 12 receptor antagonist drug treatment history or long-term use of immunosuppressive agents.
3. Recurrent myocardial infarction or previous history of Coronary Artery Bypass Graft(CABG) surgery or rescue Percutaneous Coronary Intervention.
4. Active bleeding or bleeding history.
5. With obvious infection and body temperature (axillary temperature) higher than 38.0 ℃.
6. Autoimmune diseases.
7. Malignancies.
8. In recent 6 months have received major surgery.
9. Left ventricular ejection fraction is less than 30%.
10. Life expectancy less than one year.
11. With moderate and severe liver function deterioration.
12. End-stage renal failure.
13. Other conditions that may put the patient at risk or influence study results in the investigators' opinion：eg, increased risk of bradycardiac events; known clinically important thrombocytopenia; known clinically important anemia; severe hemodynamic instability.
14. Other contraindications to investigate products.
15. Any condition that increases the risk for noncompliance or being lost to follow-up.