Purpose: The Identification of subclinical high-risk plaque is important. However, there are few data on the clinical significance of nonculprit left main coronary artery (LM) in patients with coronary artery disease(CAD). We aimed to assess the relationship between plaque composition determined by intravascular ultrasound virtual histology (IVUS-VH) and severity of coronary artery disease. Methods: In 83 patients referred for coronary angiogram, a nonculprit LM with <50% diameter stenosis was evaluated with IVUS-VH for the in-vivo coronary plaque characterization. Tissue maps were reconstructed from RF data using IVUS–VH software. Cross-sectional measurements, volumetric data of the plaque of LM and the volume of 4 different plaque components (fibrous, FI; fibrofatty, FF; dense calcium, DC and necrotic core, NC) were compared with the angiographic severity of CAD assessed by Gensini scoring system. Results: Mean Gensini score of the patients was 28짹18 and minimal lumen area of LM was 16.5짹5.4mm2. The average lumen, vessel and plaque volume of the 83 vessels were 105짹69, 150짹99 and 50짹38mm3, respectively. Mean percentage of the different plaque components were 65짹16%, FI; 15짹9%, FF; 5짹5%, DC; and 13짹9%, NC. Mean Gensini score demonstrated significant positive correlations with the volume% of DC and NC (r=0.437, p<0.001; r=0.300, p=0.006, respectively) in the LM plaque. Conclusions: In conclusion, plaque composition of IVUS-VH measurable atherosclerosis in nonstenotic LM correlated with angiographic severity of CAD. The patients with severe CAD had more percentage of NC and DC in the LM plaque. We recommend more intensive evaluation and treatment for nonculprit LM in the patients with severe CAD.