Objective To investigate the expression of miR-31 in plasm of patient with breast cancer and explore the possibility of miR-31 in plasm as a marker of beast cancer. Methods Thirty-one cases (breast cancer 21 cases, non-breast cancer 10 cases) registered in thyroid and breast surgery department of No.1 People's Hospital of Huai’an from December 2010 to March 2011 according to the selected conditions were enrolled. Five milliliters anticogulant blood were sampled to extract plasm and isolate miRNA from plasm respectively. The expression level of miR-31 in plasm were measured by real-time quantitative polymerase chain reaction（real-time qPCR）analysis. The relationship between the expression level of miR-31 in plasm and the fearutre of clinical pathology was analyzed. Result Compared with non-breast cancer, the expression level of miR-31 in plasm of patient with breast cancer was up-regulated（P﹥0.05）. The expression level of miR-31 was up-regulated in lymph positive breast cancer group compared with lymph negative breast cancer（P﹥0.05）. The expression level of miR-31 was down-regulated in tumor diameter equal or lesser than 2 centimeter compared with greater than 2 centimeter（P﹤0.05）.The expression level of miR-31 was down-regulated in stageⅠ＆Ⅱbreast cancer group compared with stage Ⅲ breast cancer（P﹥0.05）. The expression level of miR-31 was down-regulated in plasm of patient with ER and PR positive breast cancer compared with ER and PR negative breast cancer（P﹥0.05）. Conclusion This study demonstrates that the expression of miR-31 have a relationship with tumor size and could be used as potential marker of breast cancer.