Atrial fibrillation (AF) is the most common arrhythmia, characterized by a lack of atrial contraction and an irregular ventricular rhythm. We assessed the effect of fibrillatory wave amplitude on coronary blood flow in patients with AF using the thrombolysis in myocardial infarction (TIMI) frame count. Sixty-one patients with AF persisting for longer than 30 days were included. For controls, 61 age- and sex-matched patients with sinus rhythm were selected. Coarse AF was defined as any fibrillatory wave ≥ 1 mm and fine AF as any fibrillatory wave < 1 mm. Mean TIMI frame count was significantly higher in patients with AF than in those with sinus rhythm (18 ± 4 vs 30 ± 11, p < 0.001). Multivariate analysis showed that AF was the only determinant of mean TIMI frame count (β = 0.48, p < 0.001). Among patients with AF, 32 had coarse AF and 29 had fine AF. Left atrial volume index (54 ± 14 vs 64 ± 21 ml/m², p = 0.03) was significantly larger, and mean TIMI frame count (26 ± 7 vs 35 ± 12, p < 0.001) was significantly higher in patients with fine AF than in those with coarse AF. Multivariate analysis showed that hypertension (β = − 0.29, p = 0.01) and a fine fibrillatory wave (β = 0.33, p = 0.007) were determinants of mean TIMI frame count. Our data suggest that coronary blood flow is reduced in patients with AF compared with those with sinus rhythm, and that a fine fibrillatory wave is a major determinant of reduced coronary blood flow in patients with AF.