Abstract: Objective:To explore the expression and significance of Plexin-B2 in human breast cancer and to determine whether its co-expression with human epidermal growth factor receptor 2 (Her-2) is correlated with tumor aggressiveness. Methods:The pro-tein expression of Plexin-B2 and Her- 2 was investigated by immunohistochemistry SP method among 112 cases of breast carcinoma. The protein expression of Plexin-B2 was also detected among15cases of normal breast tissues. Results: There was no significant dif-ference in the expression of Plexin-B 2 between the breast carcinoma cells and the epithelial cells of the normal breast tissue from mam -mary lobules and ducts ( P > 0.05); their expression rates were 69.64% and60.00%, respectively. The positive expression of Plexin-B2 was detected in44.62% of pericancerous microvessels in the breast cancer tissues, whereas no Plexin-B2 expression was observed in the microvessels of the normal breast tissues. There was a positive relationship in Plexin-B 2 expression between the tumor cells and the pericancerous microvessels in human breast cancer ( r = 0.593, P = 0.000 ), which were correlated with clinical stage and lymph node metastases ( P < 0.05). Plexin-B 2 expression in the tumor cells was also associated with the histologic grade ( P < 0. 05). The tumors that co-expressed Plexin-B 2 and Her- 2 were characterized by worse staging and higher incidences of lymph node metastases than those that express Plexin-B 2 alone; the difference was significant ( P < 0. 05). Conclusion:The aberrant expression of Plexin-B 2 may be as-sociated with the malignant progression of breast cancer. Moreover, Plexin-B2 may contribute to the invasiveness and metastatic behav-ior of tumors when co-expressed with Her-2.