Abstract: AIM To investigete related factors of left atrial appendage (LAA) thrombus formation in patients with paroxysmal atrial fibrillation by transesophageal echocardiography (TEE) and transthoracic echocardiography (TTE). METHODS 212 patients with paroxysmal atrial fibrillation undergoing TEE were selected, and all patients were divided into LAA thrombus group (n=44) and non LAA thrombus group (n=168). General information, TTE parameters: LAD, LAEDV, LAESV, LAEF and TEE parameters: LAA-v, LAA width (LAA-W) and LAA depth (LAA-D) at 45°90°135°in detail, and Binary Logistic Regression Analysis was used to explore related factors. RESULTS No significant differences were found between the two groups at age, sex, proportion of hypertention, diabetes and hyperlipidemia, and LAA-W, LAA-D at 45°90°135°. The proportion of coronary heart disease, LAD, LAEDV and LAESV were higher, but LAEF and LAA-v were lower in the thrombus group (P<0.05). Binary Logistic Regression Analysis demonstrated that LAEF (OR=1.163, 95%CI: 1.043-1.296, P<0.01) and LAA-v (OR=29.82, 95%CI: 7.97-29.82, P<0.05) were independent risk factors. CONCLUSION Thrombosis of left atrial appendages in patients with paroxysmal atrial fibrillation are closely related to LAEF and LAA-v.