Objective To investigate the relationship between serum cathepsin S (Cat S) and cystatin C(Cys C) expression levels with carotid arterial plaque property in the patients with ischemic stroke(IS).Methods A total of 336 cases of atherosclerotic cerebral infarction were divided into the stable plaque and unstable plaque group according to the carotid arterial ultrasound.Contemporaneous 114 individuals undergoing healthy physical examination served as the control group.Fasting blood was collected from all subjects entering the groups for detecting serum Cat S and Cys C.Results The serum Cat S and Cys C levels and Cat S/Cys C ratio (Cat S/Cys C) had statistically significant difference between the two groups (P＜0.05),moreover the Cat S level in the unstable plaque group of the patients with cerebral infarction was increased compared with that in the stable plaque group,while serum Cys C level was decreased compared with that in the stable plaque group (P＜0.05).The Cat S levels were (75.34-±-15.45)pg/mL and (60.12±18.53)pg/mL and the Cys C levels were (0.73±0.62)mg/L and (0.93±0.53)mg/L,the Cat S/Cys C ratios were (103.68±2.52) and (64.64± 9.24) respectively,the difference between the two groups had statistical significance (P＜0.05).The Spearman correlation analysis showed that the serum Cat S level and Cat S/Cys C ratio had obviously positive correlation with carotid arterial unstable plaque (r=0.498,P＜0.05;r=0.753,P＜0.01);while the level of serum Cys C was negatively correlated with the unstable plaque (r=-0.213,P＜0.05).Conclusion Serum Cat S/Cys C level has a certain correlation with carotid arterial plaque property in ischemic stroke patients,which may become the serological indicators for predicting carotid arterial plaque.