Objective To explore the impact of Danhuang Tongmai Decoction（ 丹黄通脉方） on carotid atherosclerotic plaque and the possible mechanism. Methods Thirty Sprague Dawley（ SD） rats were randomized into a blank group

a model group and a Danhuang Tongmai Decoction group

10 rats of each group. The model group and the Danhuang Tongmai Decoction group were established carotid artery atherosclerosis models by high fat compound feed and intraperitoneal injection of vitamin D3. After the models were 〗 successfully established

the Danhuang Tongmai Decoction group were given Danhuang Tongmai Decoction 10 ml/（ kg·d） by gavage; the blank group and the model group were given equivalent distilled water

twice every day

for 6 weeks. Arteria carotis pathological morphological changes of rats in all groups were examined. The levels of matrix metalloproteinase 9（ MMP-9） and matrix metallo-proteinase inhibitor 1（ TIMP-1） were detected. Results Observation under light microscope showed that the tunica intima in the model group was significantly thickened

with foam like cell accumulation

varying degrees of damage and fracture of elastic fiber layer. The tunica intima in the Danhuang Tongmai Decoction group was a little thickened

with a small amount of foam like cell form and slightly disorder of elastic fiber layer

which was less serious than those in the model group. Compared with that in the blank group

the ratio of carotid intima and tunica media（ I/M）

and levels of MMP-9 and TIMP-1 in the model group and the Danhuang Tongmai Decoction group increased（ P < 0. 05）. Compared with that in the model group

I/M and serum levels of MMP-9 in the Danhuang Tongmai Decoction group decreased（ P < 0. 05 or P < 0. 01）. Conclusion Danhuang Tongmai Decoction might down-regulate the level of MMP-9

which might be one of the mechanisms of improving carotid artery atherosclerosis and stabilizing plaque.