Objective　To investigate the expression of Second mitochodria-derived activator of caspase (Smac) and Caspase-9 in esophageal squamous cell carcinoma (ESCC) and its relationship with apoptosis. Methods　Immunochemistry(SP-method) was applied in semi-quantitive analysis on protein of Smac and Caspase-9 in cancer tissues, normal tissues and adjacent tissues of 80 cases of ESCC. Apoptosis of tumor cells in paraffin sections was observed with the method of TUNEL.　Results　(1) The positive expression rate of Smac were 92.50% , 63.75%and 77.50% in normal tissues, ESCC tissues, adjacent tissues and respectively, the positive expression rates of Caspase-9 were 85.00%, 72.50% and 58.75% respectively in normal tissues, ESCC tissues and adjacent tissues. The differences were of statistical significance. (2) 47 cases with Caspase-9 positive expression were detected in 51 cases of ESCC with positive expression of Smac, accounting for 92.16%; the correlation between Smac and Caspase-9 expression was strong and positive. (3) The average apoptotic index (AI%) in Smac positive group (19.186±1.331) was apparently higher than that in Smack negative group (8.316±1.661), which has no significant difference from the Caspase-9 positive expression group. The average apoptotic index of 47 cases with positive co-expression of Smac and Caspase-9 was statistically significantly higher than that with negative expression of Smac and Caspase-9.　Conclusions　The expression of Smac and Caspase-9 protein are positively related and both of them are involved in the genesis and development of ESCC, which also play a positive feedback regulation in the apoptosis of ESCC cells.