Aim To identify the correlation between chitinase protein 40 (YKL-40) levels including high sensitive C reactive protein (hs-CRP) and the fibrous cap thickness of fibrofatty plaque in coronary culprit 1esions. Methods Clinical data of 60 patients with selective coronary artery angiography diagnosed coronary artery disease were retrospectively analyzed. According to type of coronary disease, patients were divided into 3 subgroups:SAP group (containing 22 stable angina patients), UAP group (containing 28 unstable angina patients), and AMI group (containing 10 acute myocardial infarction patients). Serum hs-CRP and YKL-40 levels were measured before subsequent procedures. The characteristics of the culprit lesions were detected by optical coherence tomography (OCT) before interventional treatment, and the correlation between hs-CRP, YKL-40 and the fibrous cap thickness of fibrofatty plaque in coronary culprit lesions were analyzed. Results (1)The serum levels of hs-CRP and YKL-40 were significantly higher in AMI group than in SAP and UAP group (all P<0.05), and higher in UAP group than in SAP group (all P<0.05). (2)The fibrous cap thickness of fibrofatty plaque in coronary culprit lesions were smaller in AMI and UAP group than in SAP group (all P<0.05), and there was no significant difference between AMI group and UAP group (P>0.05). Proportion of thin-cap fibroatheroma plaque (all P<0.05), plaque rupture and thrombosis were significantly higher in AMI group than in SAP and UAP group (all P<0.05). Proportion of calcification in plaque was lower in AMI group than in SAP group (P<0.05), and there was no significant difference between AMI group and UAP group (P>0.05). (3)Pearson correlation analysis showed that serum levels of hs-CRP (r=－0.265, P<0.05) and YKL-40 (r=－0.524, P<0.01) were negatively correlated with fibrous cap thickness of fibrofatty plaques. Spearman correlation analysis showed that serum levels of hs-CRP and YKL-40 were positively correlated with plaque rupture (r=0.462 and r=0.499, P<0.01) and thrombosis (r=0.218 and r=0.263, P<0.05). (4)Multiple Logistic regression analysis showed that serum levels of YKL-40 at baseline was independently related to thin-cap fibroatheroma plaque (OR=6.341, P<0.01). Conclusions The serum levels of hs-CRP and YKL-40 in AMI patients were much higher than that in SAP and UAP patients, higher in UAP patients than in SAP patients. Prevalence of thin-cap fibroatheroma plaque, plaque rupture and thrombosis was significantly higher in the AMI patients, while the prevalence of calcification in plaque was more often in SAP patients. Increased serum levels of YKL-40 were independent risk factor of thin-cap fibroatheroma plaque formation.