Objectıve: The aim was to compare coronary high-definition CT (HDCT) with standard-definition CT (SDCT) angiography as to radiation dose, image quality and accuracy. Material and methods: 28 patients with history of coronary artery disease scanned by HDCT (Discovery CT750 HD) and SDCT (Somatom Definition AS). The scan modes were both axial prospective ECG-triggered. The vessel diameters and vessel attenuation values of totally 280 measurements from 140 coronary arteries were analyzed by two experienced radiologists. All data was analyzed by intraclass correlation test. Image quality graded by motion and stair step artifacts (grade 1, poor, to grade 4, excellent), accuracy of vessel inner and outer diameters were compared between the two CT units using the independent samples t-test and Mann-Whitney U test. Results: The intraclass correlation coefficient (ICC) of measured vessel attenuation values in SDCT between the two radiologists was exceedingly good. The ICC was higher in HDCT. The radiation dose of HDCT was higher than that of SDCT. The mean tube current was 180 (mA) in HDCT and 147(mA) in SDCT with the same tube voltage (kVp). There was no significant difference between image quality. Conclusion: HDCT has a higher radiation dose but has much more atenuation and the spatial resolution which improve measurement accuracy for imaging coronary arteries. Sci. 2015;15(3):925-30. doi: http:// dx.doi.org/10.4314/ahs.v15i3.29