To quantitatively study various morphologic parameters of the left atrial appendage (LAA) by computed tomography (CT) to aid the preoperative planning and implantation of left atrial appendage closure devices. In 612 cases of patients with or without atrial fibrillation (AF), a cardiac CT study was performed. The classification of general LAA morphology included ChickenWing type (18.3%), WindSock (46.7%), Cauliflower type (29.1%), and Cactus type (5.9%). Anatomical relationship of the LAA to the left superior pulmonary vein (LSPV) were classified as high type (superior to LSPV, 30.2%), mid type (parallel to LSPV, 58.1%), and low type (inferior to LSPV, 11.7%). LAA ostium could be classified into 5 types including oval (68.9%), foot-like (10%), triangular (7.7%), water drop-like (7.7%), and round (5.7%). Two-dimensional (2D) orthogonal method was obviously not accurate for determining the LAA orifice because the measurement was often unparallel to the LAA orifice. Two-dimensional oblique method was better than 3-dimensional method in reproducibility to determine the size of LAA ostium. The diameter calculated from the perimeter of the LAA ostium was superior to the diameter from direct measurement of the LAA ostium for selecting the occluder. The morphology of the LAA and the LA ostium are extremely complex and heterogeneous. Sixty-four-channel cardiac CT could assist preoperative planning of LAA closure device placement. The diameter of the LAA ostium calculated from the perimeter is the best parameter for sizing the LAA occluder. © 2010 Wiley Periodicals, Inc.