Abstract: To investigate the expression of DNA mismatch repair gene hMLH1 and its clinical significances in small bowel adenocarcinoma(SBA). Methods hMLH1 expression was detected on cancerous and adjacent normal tissues from 36 cases of SBA using IHC (SP method). Results Positive rate of hMLH1 protein was lower in SBA (66.7%) than in tumor-adjacent tissuses (91.7%). Significant difference was found between the expression of cancerous and adjacent normal tissues. The hMLH1 protein expression was significantly correlated with the degree of tumor cell differentiation (P＜0.05),but no correlation with other clinicopathologic parameters such as age, sex,tumor size,location,invasive depth and lymph node metastasis (P＞0.05 ). Conclusion There was a subset of SBA accompanying MMR defect, hMLH1 gene can be used as a target to evaluate the tumor differentiation.