The cause of prolonged filtered P-wave duration (FPD) remains unclear in atrial fibrillation (AF) patients with normal left atrial size. We investigated whether FPD is associated with left atrial pressure (LAP) in AF patients without prominent LA enlargement. This study included 80 patients (48 men, age 65 ± 9 years, 25 persistent AF) with non-valvular AF who underwent catheter ablation (CA) for AF. LAP was measured in sinus rhythm during CA and signal-averaged electrocardiogram was recorded after CA. We retrospectively assessed the clinical and echocardiographic variables. Prolonged FPD was defined as FPD > 120 ms. Prolonged FPD (FPD > 120 ms) was detected in 23/80 patients (29 %). According to univariate analysis, higher mean LAP (14.9 ± 4.4 vs. 10.8 ± 3.5 mmHg, p < 0.0001), higher prevalence of persistent arrhythmia, higher BNP, larger LAD, higher E wave, and lower LVEF were associated with Prolonged FPD. According to multivariate analysis, higher mean LAP was the only factor associated with Prolonged FPD (p = 0.0058, OR 1.256 for each 1 mmHg increase in mean LAP, 95 % CI 1.068–1.476). Moreover, a significant correlation was observed between FPD and mean LAP (r = 0.503, p < 0.0001). Prolonged FPD is associated with high LAP in AF patients without prominent left atrial enlargement. Pressure overload of the left atria might cause slowing of atrial electrical activation.