Objective: To evaluate the association between the ratio of early diastolic transmitral velocity to early diastolic mitral annular velocity (E/E') and left atrial pressure (LAP) estimated from invasive catheter measurements in patients with atrial fibrillation (AF). Methods: A total of 46 consecutive patients with non-valvular AF and preserved left ventricular ejection fraction (LVEF) admitted in our department to receive the first radiofrequency ablation from May to July 2017 were included. All patients underwent echocardiography at 24-48 hours before radiofrequency ablation, and LAP was invasively measured during the ablation procedure. According to mean LAP, patients were divided into 2 groups of normal LAP (LAP≤12 mmHg(1 mmHg=0.133 kPa, n=31) and elevated LAP (LAP>12 mmHg, n=15). Linear correlation analysis was used to evaluate the relationship between E/E' and LAP. Results: E/E' correlated well with LAP (septal E/E' (E/E'_sep), r= 0.397, P=0.006; lateral E/E' (E/E'_lat), r=0.433, P=0.003; mean E/E' (E/E'_mean), r=0.431, P=0.003). Using receiver operating characteristic analysis, the optimal cut-off for E/E'_sep was 12.5 (sensitivity 73.3%, specificity 67.7%), E/E'_lat was 10.8 (sensitivity 80.0%, specificity 77.4%), E/E'_mean was 11.0 (sensitivity 86.7%, specificity 64.5%) to predict mean LAP>12 mmHg. Conclusion E/E', especially the E/E'_lat, is positively correlated with LAP in patients with AF and preserved LVEF, and may be used to estimate the diastolic function in AF patients with preserved LVEF.