Objective To evaluate the accuracy of delayed scanning of 256-slice CT coronary angiography (CCTA) in the diagnosis of left atrial auricular(LAA) thrombosis in patients with atrial fibrillation(AF), confirming the value and necessity of delayed scanning. Methods 87 patients with AF for CCTA scanned on 256-row CT (GE Revolution) were enrolled in the study. All patients were scanned in one heart cycle with gantry rotation time of 0.28s, kV Assist and Smart-mA with noise index of 25HU. The phase of the CCTA scan was determined by Auto-gating technique with an exposure time of 0.5 to 0.8s. The contrast-media (370 mg I/ml) was injected at a flow rate of 0.08 ml/s/kg for 10 seconds and 50ml saline solution. With Bolus-Tracking technology, the CCTA scan was started with a delay of 4 seconds after the ascending aorta threshold reached 300 HU. After the CCTA scan, a radiologist immediately viewed the image on the console to determine the filling of left auricula, delayed scan only for incomplete filled patients at 60s after CCTA scan. The scanning phase was set to 250ms to 550ms after the R wave, the exposure time is 0.3s, and the noise index is 30HU. CTDIvol and DLP for CCTA scan and delayed scan were recorded. Results Of the 87 patients with CCTA scan, the number of filled or incomplete filled of LAA were 34 and 53 respectively. Of the 53 patients who underwent delayed scans, 22 patients with LAA filling defects were diagnosed LAA thrombosis by CT, TEE confirmed 20 cases of thrombosis, and the other two cases thrombus less than 5 mm were missed. The proportion of patients with LAA incomplete filled during CCTA scan was 60.9%(53/87), and the proportion of patients with who were diagnosed LAA thrombus with delayed scan was 25.3%(22/87). Conclusion Delayed scanning can significantly improve the accuracy and reliability of LAA thrombosis in 256- row CT coronary angiography in patients with atrial fibrillation.