Background There are few reports of quantitative and qualitative measuring of left main coronary artery (LMCA) plaques by multislice computed tomography coronary angiography (MSCTA), especially when compared with intravascular ultrasound (IVUS) as reference standard. The aim of this study was to evaluate the use of 64-MSCTA in the diagnosis of LMCA disease, and the accuracy of MSCTA in the quantitative and qualitative assessment of the LMCA lesion as compared with IVUS.Methods A total of 91 patients (53 men, 38 women, mean age (64.78±9.19) years) were examined by 64-MSCTA and IVUS. Compared with the IVUS, the sensitivity, specificity, positive and negative predictive values (PPV and NPV) of the MSCTA on the diagnosis of LMCA diseases were calculated. Also, kappa index (K) for the agreement between MSCTA and IVUS was calculated. Minimal lumen area (MLA), extal elastic membrane cross-sectional area (EEM-CSA) and plaque burden were measured by two blinded and independent operators on MSCTA cross-sectional reconstruction and compared with the parameters measured from IVUS by manually tracing. The CT value of soft, fibrous and calcific plaques was measured using IVUS classification of the plaques.Results The sensitivity, specificity, PPV and NPV of MSCTA for detecting LMCA plaques were 93.1%, 84.2%, 95.7%, 76.2%, respectively. Kappa index (K=0.744, P<0.001) indicated excellent agreement between MSCTA and IVUS. The Pearson index between MLA on IVUS and MLA on MSCTA was 0.815 (P <0.01). The Pearson index of plaque burden and EEM-CSA between IVUS and MSCTA was 0.736 and 0.740 respectively (both P <0.01). The CT value of soft plaque, fibrous plaque and calcific plaque compared with IVUS were (52.52±15.71) HU, (108.32±43.44) HU and (604.16±377.67) HU (P<0.001). Receiver operating characteristic curve analysis of CT value of non-calcific plaques for predicting soft plaques showed the cutpoint was 54.35 HU, with a sensitivity of 83.3% and specificity of 94.4%. Conclusions Sixty-four section MSCTA is an effective diagnostic tool for the detection of LMCA plaques with higher sensitivity and specificity. The correlation of quantitative and qualitative analysis between MSCTA and IVUS was excellent. The CT value of plaques can help the diagnosis of plaque composition.