The purpose was to investigate the influence of shorter delay time on the interpatient variability in coronary enhancement and appropriateness of scan timing in coronary CT angiography (CTA) using bolus tracking method with 320-row CT. The bolus tracking scan was performed at the level of the bifurcation of the trachea for first 50 patients (group 1) and at the center level of the diagnostic scan for the last 50 patients (group 2). The CT number of the proximal coronary arteries was measured in the right coronary artery (RCA) and the left main trunk (LMT). The CT numbers of the right ventricle, left ventricle, ascending aorta, and descending aorta were also measured to consider the appropriateness of the scan timing. The delay time was longer in group 1 than in group 2 (7.0 vs. 2.6 s; p < 0.0001). The CT number within the RCA was 390 ± 75 HU for group 1 and 419 ± 42 HU for group 2. The CT number within the LMT was 396 ± 72 HU for group 1 and 420 ± 40 HU for group 2. The difference of average (p = 0.02 and 0.04) and standard deviation (p = 0.03 and 0.02) was statistically significant. The scan timing was early or late in 15 patients for group 1, but only 2 patients for group 2 (p = 0.0002). Shortening the delay time could reduce the interpatient variability in coronary enhancement with appropriate scan timing in coronary CTA.