Abstract: Objective: To study the relationship and morphological assessment of left atrial appendage (LAA) and adjacent anatomical structure by SOMATOM Definition Flash CT. Methods: 83 patients were selected retrospectively including 62 patients with sinus rhythms underwent coronary angiography and 21 patients with atrial fibrillation underwent left atrial enhancement scanning. The relationship between the LAA and the pulmonary vein was observed, and the diameter of the left atrial appendage was measured quantitatively. Then the data between the two groups were compared. Results: There was no significant difference in pulmonary vein variation rate and anatomical relationship of the LAA to left superior pulmonary vein (LSPV) between the atrial fibrillation patients and the control group (P > 0.05). And there were significant differences in the length, area, volume and the first turning angle of LAA in the two groups (P < 0.05). Conclusion: SOMATOM Definition Flash CT scan can quantitatively measure the diameter of LAA, and it can also clearly show the correlation of LAA and its adjacent anatomy. So it may have an important guiding significance for atrial fibrillation ablation and LAA occlusion.