Background: Previous trials using intravascular ultrasound(IVUS) have demonstrated that the lipid lowering therapy with statin decrease plaque volume and increase plaque echogenecity in patients with coronary artery disease. The aim of this study was to demonstrate the early effect of statin on plaque composition by IVUS-Virtual Histology(VH) during half a year after coronary event and to examine the influence of changes in lipid profile and CRP on the plaque composition. Methods: Thirty six patients with ACS were prospectively enrolled. One target segment was determined in a non-PCI site (>5 mm proximal or distal to the PCI site) with a reproducible index side branch or from the ostium of target vessel. We classified and analyzed the target plaques into two types according to plaque stability: thin-cap fibroatheroma (TCFA, n=10); and non TCFA(n=26) Results: In 6 months after statin therapy, no change was demonstrated in mean percentage of the lipid core(16.9 짹 8.5% to 16.2 짹 10.1%, p=0.67), whereas plaque volume and LDL-cholesterol level were significantly reduced. There was a significant reduction of lipid core percentage in the TCFA type (21.1 짹 7.6 % to 10.9 짹 5.8 %, p= 0.001), but not in the non-TCFA type. The percentage change in lipid core showed a significant relation with change in high-sensitivity C-reactive protein(r=0.4, p=0.006). Changes in LDL cholesterol and lipid core demonstrated no significant associations. Conclusions: A clear reduction of lipid core was observed only in the TCFA type which suggest that the changes of plaque composition following statin therapy might occur earlier in vulnerable plaque than stable plaque and appears related to their anti-inflammatory effects.