Abstract: Objective: To investigate the expression of decorin (DCN) in breast carcinoma. Methods: The immunohistochemistry and in situ hybridization technique were used to detect the expressions of DCN protein and mRNA in the breast cancer tissues of 38 cases and related normal mammary tissues of 9 cases. Results: The expression of DCN mRNA can be found in cancer cells of the breast cancer, with a positive rate of 88.00% (22/25), and it can not be found in glandular cells of the normal breast. There was a significant difference between the two (P<0.01). The rate of positive expression of DCN protein in 38 cases was 55.26%(21/38) and there was no expression of DCN protein in glandular cells of the normal breast. So there was a significant difference between the two (P<0.01). The expression of mesenchymal DCN protein in breast cancer was higher than that in normal mammary tissue, so the difference was significant between them (P<0.01). Conclusion: The expressions of DCN mRNA and protein in cancer cells and mesenchyme of the breast cancer were higher than that in related normal mammary tissue.