Objective:To investigate the expression of CD40 and MMP9 in carotid atherosclerotic plaques, so as to assess the role of CD40 in the stability of the plaque and the possible mechanism. Methods: The expression of CD40 and MMP9 mRNA and protein in carotid atherosclerotic plaques obtained from carotid eversion endarterectomy (CEE) of 28 patients with high-grade stenosis (>70%) (stroke group, n=15; non-stroke group, n=13) and 8 normal postmortem arteries (control group) were detected by real-time quantification polymerase chain reaction(PCR) and Western blotting. The correlation between expression of CD40 and MMP9 was analyzed. Results: The expression of CD40 mRNA and MMP9 mRNA in the non-stroke group and stroke group was significantly higher than that in control group (P＜0.01); and that of the stroke group was significantly higher than that of the non-stroke group (P＜0.01). There was a linear correlation between expression of CD40 and MMP9 mRNA (r=0.964，P＜0.01). Extremely rare expression of CD40 and MMP9 protein was found in the normal carotid artery; the protein expression was evidently higher in the carotid artery atherosclerosis than in the normal carotid artery, and the expression in carotid artery atherosclerosis in the stroke group was higher than that in the non-stroke group. Conclusion: The increased expression of CD40 and MMP9 in the carotid atherosclerositic plaques may be closely related to the stability of plaques.