Abstract: This study aimed at evaluating the antioxidant effects of Guanfu base A(GFA)on acetylcholine(Ach)/CaCl₂(CaCl₂ 10 mg/mL, Ach 66 μg/mL)-induced atrial fibrillation(AF)in rats. SD rats were rando-mized into normal group, model group, GFA treatment groups(6 mg/kg, 12 mg/kg), Amiodarone(Ami)treatment group(50 mg/kg)and Lovastatin(Lov)treatment group(10 mg/kg). The AF durations were measured by electrocardiogram(ECG). The effective refractory periods(AERP)were measured in the left atrial appendage. Oxidative stress-related gene and protein expression was evaluated by RT-PCR and Western blot. The activity of antioxidant enzymes was measured by enzymatic assay. Results indicated that, in comparison with that in the vehicle-treated AF rats, treatment with GFA(6 mg/kg, 12 mg/kg, po), significantly shortened the AF duration and prolonged the AERP in rats. In addition, treatment with GFA reduced the levels of plasma and myocardium malondialdehyde, increased the activity of plasma superoxide dismutase in a dose-dependent manner. Moreover, treatment with GFA mitigated AF-up-regulated p22phox, p47phox, gp91phox, and p67phox NADPH oxidase expression, and AF-increased ratios of membrane to cytosolic Rac-1 in the atrium. It also significantly prevented AF-down-regulated atrial connexin40 expression in rats. Data suggested that GFA(6 mg/kg, 12 mg/kg)has potent anti-oxidant activity and inhibits oxidative-stress-related AF in rats.