Background Non-valvular atrial fibrillation is associated with an increased risk of ischemic stroke; however,the appropriate intensity of anticoagulation therapy for Chinese patients has not been determined.The purpose of this study was to compare the safety and the efficacy of standard-intensity warfarin therapy,low-intensity warfarin therapy,and aspirin therapy for the prevention of ischemic events in Chinese patients with non-valvular atrial fibrillation (NVAF).Methods A total of 786 patients from 75 Chinese hospitals were enrolled in this study and randomized into three therapy groups:standard-intensity warfarin (intational normalized ratio (INR) 2.1 to 2.5) group,low-intensity warfarin (INR 1.6 to 2.0) group and aspirin (200 mg per day) group.All patients were evaluated by physicians at 1,3,6,9,12,15,18,21 and 24 months after randomization to obtain a patient questionnaire,physical examination and related laboratory tests.Results The annual event rates of ischemic stroke,transient ischemic attack (TIA) or systemic thromboembolism were 2.6％,3.1％ and 6.9％ in the standard-intensity warfarin,low-intensity warfarin and aspirin groups,respectively (P=0.027).Thromboembolic event rates in both warfarin groups were significantly lower than that in the aspirin group (P=0.018,P=0.044),and there was no significant difference between the two warfarin groups.Severe hemorrhagic events occurred in 15 patients,7 (2.6％) in the standard-intensity warfarin group,7 (2.4％) in the low-intensity warfarin group and 1 (0.4％)in the aspirin group.The severe hemorrhagic event rates in the warfarin groups were higher than that in the aspirin group,but the difference did not reach statistical significance (P=0.101).The mild hemorrhagic and total hemorrhagic event rates in the warfarin groups (whether in the standard-intensity warfarin group or low-intensity warfarin group) were much higher than that in the aspirin group with the annual event rates of total hemorrhages of 10.2％,7.6％ and 2.2％,respectively,in the 3 groups (P=0.001).Furthermore,there was no significant difference in all cause mortality among the three study groups.Conclusion In Chinese patients with NVAF,the warfarin therapy (INR 1.6-2.5) for the prevention of thromboembolic events was superior to aspirin.