Background: There have been few in vivo data on the differences of plaque composition between diabetic and non-diabetic patients. The purpose of our study is to analyze plaque composition of culprit lesion using IB-IVUS System (YD Ltd, Japan) and to compare the results between diabetic and non-diabetic patients presented with acute coronary syndrome (ACS). Methods: Sixty three significant de novo coronary lesions in diabetic (n=25) and non-diabetic (n=38) ACS patients were evaluated with a 30 MHz phased-array IVUS catheter with motorized pull-back system at 0.5 mm/s (Galaxy II, Boston Scientific, USA) prior to percutaneous coronary intervention (PCI) with stenting. Tissue characterization of target lesions by IB-IVUS was based on radiofrequency backscatter signal analysis. Radiofrequency signals were classified into four component types: calcification, fibrotic tissue, mixed tissue, and lipid pool. For each cross section image, the content of these plaque components was assessed by an integrated soft ware. Total and mean content values for each tissue component from all the image slices for each lesion were obtained. Chronic total occlusion, bifurcated or heavily calcified coronary lesions were excluded. Results: Baseline characteristics, laboratory findings with the exception of glucose level and lipid profile, and conventional IVUS parameters such as plaque area and lumen area showed no significant difference between the two groups. However, the diabetic group showed a larger amount of lipid pool content compared to the non-diabetic group (38.9짹 10.4% vs. 32.7짹 10.3% p=0.045). Other plaque components such as calcification, fibrotic or mixed tissue showed no significant difference. Conclusions: Based on the analysis by IB-IVUS, the culprit lesion of diabetic patients contained larger amount of lipid pool compared to those of non-diabetic patients. Clinical implication of higher lipid pool content in target lesion of diabetic patients needs to be investigated in further studies.