Although left atrial volumes (LAVs) and transport function can be accurately measured by multi-slice computed tomography (MSCT) during sinus rhythm, limited data are available for in patients with atrial fibrillation (AF). The aims of our study were to compare LAVs and function assessed by MSCT and transthoracic echocardiography (TTE) during AF, and to validate a simplified method to determine LAVs and functions using MSCT. A total of 150 consecutive AF patients who were scheduled to undergo catheter ablation were enrolled in this study. All subjects underwent MSCT and TTE on the same day. LAVs were measured by MSCT at every 10% of the R–R interval (10-phase analysis). LA transport function was assessed by measuring changes in LAVs. LAVs and functions were also assessed by TTE using a modified Simpson’s method and an area-length method. Fifty patients were in sinus rhythm (SR) and 100 were in AF during their examinations. Although TTE underestimated the maximal LAV (LAV_max, by 38.3%) and overestimated the total LA emptying fraction (LAEF_total, by 61.1%) compared with MSCT, there were excellent correlations between TTE and MSCT. LAV_max and the minimal LAV (LAV_min) based on MSCT were determined at relatively constant cardiac phases during AF as well as SR (LAV_max; 40%, LAV_min; 100% of R–R interval, fixed-phase analysis). LAVs and LAEF_total assessed by 10-phase analysis showed good correlations with those assessed by fixed-phase analysis (P < 0.001). LAVs and function assessed by MSCT correlated well with LAVs and functions assessed by TTE, irrespective of the underlying rhythm. Our results indicate that in the context of MSCT, fixed-phase analysis is a simple and reliable method to determine LAVs and function in patients with AF.