Objective To define the optimal enhancement scanning triggering threshold(ESTT)for both coronary and myocardial imaging of dual energy coronary computed tomography angiography (DE-CCTA).Methods One hundred of 135 patients who were going to receive DE-CCTA were enrolled in this study prospectively and equally assigned to five groups randomly of different ESTT as 100, 110, 120, 130 and 140 HU. Noise, signal-to-noise ratios (SNR), contrast-to-noise ratios (CNR), enhancement extent (EE) of coronary artery and myocardium, iodine concentration, beam hardening (BH) objective value and subjective rating of myocardium and EE of left ventricle (LV) were measured and compared between different group;and noise,SNR,CNR,EE of coronary artery and myocardium,iodine concentration and BH objective value of myocardium obey normal distribution and equal variance,so they were expressed as mean± standard deviation and compared with variance analysis of multiple independent samples, and pairwise comparison were done with SNK test.Myocardial BH subjective rating was ranked data,and was expressed as [median (P25, P75)], and compared with Kruskal-Wallis test, and pairwise comparison were done with Nemenyi test.Results Post contrast CT value of LAD,post contrast CT value and EE of LV chamber were different with different ESTT (F=3.471, 3.795, 3.132,and all P<0.05)Post contrast CT value of LAD was lower with 130 HU[(355.16±59.11)HU than ESTT of 100 HU[(404.9±49.0)HU].Myocardial BH value and subjective score were different with different ESTT(F=6.118,H=13.702,P all<0.05).Myocardial BH value was lower with ESTT of 130,140 HU[(8.8±12.4),(5.4±17.1)HU]than 100,110 HU[(24.2±17.7),(22.1± 11.6)HU].Myocardial BH subjective score was lower with ESTT of 130 HU[1.5(1.0,2.0)]than 110 HU[3.0 (2.0, 3.0)]. Post contrast CT value and EE of LV myocardium, noise, SNR and CNR of LAD and LV myocardium, EE of LAD were not different between group of different ESTT (P all>0.05). Conclusions ESTT could influence image quality of myocardium and coronary artery.One hundred and thirty HU is optimal ESTT of DE-CCTA for showing myocardium and coronary artery simultaneously during DE-CCTA on second generation dual-source CT with retrospective electrocardiogating technic and as monitoring position at aortic root.