Aim To investigate the effect of rat peritoneal mast cell on collagen Ⅰand Ⅲ expression in foaming smooth muscle cell(SMC). Methods SMC were treated with the supernatant of mast cell completmented with 100mg/L oxidized low density lipoprotein (ox-LDL)for 48h. The mRNA expressions of collagenⅠand Ⅲ were detected by RT-PCR. Immunocytochemistry staining was used to detect the protein expression of collagen Ⅰand Ⅲ. Results With exposure to mast cell supernatant and ox-LDL, the expression of collagen Ⅰand Ⅲ in SMC decreased. Conclusions Mast cell inhibits SMC collagen Ⅰand Ⅲ expression in vitro. Our findings support the hypothesis that mediums released from activated mast cell in atherosclerotic plaques contributes to cap remodeling.