This study was designed in order to evaluate and compare the efficacy and safety between aspirin and clopidogrel in the patient with low stroke risk Atrial Fibrillation (AF).

Efficacy outcome will be evaluated the major cerebro-cardiovascular event including stroke, cardiovascular death, and myocardial infarction during 1 year-medication period. In addition, safety outcome will be evaluated the gastrointestinal responses including peptic ulcer and upper gastrointestinal bleeding events to both study drugs by repeated gastroenteroscopic examinations before and after medication by GI specialist.

Inclusion Criteria:

1. Man or Women > 20 years old
2. newly detected AF (CHA2DS2VASc index score: 1)
3. Patient who needs antiplatelet therapy using aspirin clopidogrel for stroke prevention
4. volunteer only
5. childbearing aged women who takes proper oral contraceptive

Exclusion Criteria:

1. No specific contraindication or any history of hypersensitivity of Clopidogrel or aspirin
2. Patient with active GI bleeding or bleeding tendency or major bleeding history
3. less than 1 year of residual expected life
4. Pregnant or breast-feeding women
5. Other causes, determined by charged physician
6. Patient with definite GERD who needs special treatment
7. Patient who needs to take NSAID (Non-Steroidal Anti-Inflammatory Drug) for more than 2 weeks