Abstract: 【Abstract】 Objective To analyze the dynamic change of regulatory T cells in experimental murine mammary carcinoma model，and to investigate their effect on tumor progress. Methods Mouse mammary carcinoma models were established. The percentages of CD4+CD25+ and CD4+ FOXP3+regulatory T cells in the CD4+T cells in tumor tissue, tumor draining lymph node and spleen were measured by FACS at 3 different time points after tumor challenge. The expression of CD25+FOXP3 in CD4+ T cells was analyzed, and the expression of CD4+ T cells in T cells （CD3+）of tumor tissue. Results Compared with the normal control group, In mouse mammary carcinoma models, The percentages of CD4+ T cells to lymphocytes in lymph node and spleen were decreased in the late stage （P0. 05）. Conclusion Regulatory T cells were significantly increased in malignant tumor model, and closely related to the development of tumor.