The clinical significance of the transluminal attenuation gradient (TAG) has not been established. We evaluated the incremental diagnostic value of TAG by 320-row area detector computed tomography (320-ADCT). Subjects were 65 patients who underwent one-rotation scanning by 320-ADCT and invasive coronary angiography (ICA) within 3 months. TAG values were obtained for the major epicardial vessels 2 mm or more each in RCA, LAD and LCX using automatic analysis software. Moreover, TAG values that excluded calcified lesions in calculation of the regression line were also evaluated (excluded-TAG). In LAD, 21 intermediate lesions underwent functional flow reserve (FFR), and the incremental diagnostic value for functional stenosis was evaluated. The TAG values in the normal vessels were − 8.3 ± 5.0 (HU/cm) for the RCA (n = 32), − 23.3 ± 4.3 for the LAD (n = 9) and − 20.6 ± 10.0 for the LCX (n = 32). The RCA value was significantly higher (p < 0.001). The TAG values with stenosis degrees of ≤ 25%, 26–75%, ≥ 76% on ICA were − 8.3 ± 5.0 (n = 32) vs − 10.3 ± 7.2 (n = 25) vs − 10.0 ± 5.4 (n = 4) in the RCA, − 23.3 ± 4.3 (n = 9) vs − 21.0 ± 11.5 (n = 35) vs − 23.5 ± 15.3 (n = 10) in the LAD and − 21.1 ± 15.1 (n = 32) vs − 21.1 ± 15.1 (n = 16) vs − 17.7 ± 15.7 (n = 6) in the LCX, with no significant difference among the three groups. The excluded-TAG values also showed no significant difference. The area under the curve in the diagnosis of FFR < 0.8 in 21 LAD cases was 0.542 for CT only, 0.694 for CT + TAG, and 0.694 for CT + excluded-TAG. In single time-phase scanning by 320-ADCT, TAG does not offer an incremental diagnostic value.