Objective: To explore the expression of PLK1 in the molecular subtypes of breast carcinoma and their relationship with basal-like breast carcinoma. Methods: A total of 803 cases of invasive ductal breast carcinoma were identified, the patients were subclassified in Luminal A, Luminal B, HER-2 over-expressing, basal-like and normal breast-like subtypes according to Nielsen criteria. The expression of PLK1 in five subtypes of invasive ductal breast carcinoma and the relationship between the expression of PLK1 and the clinicopathologic features of basal-like breast cancer were detected by immunohistochemistry. Results: Positive expression rate of PLK1 in basal-like, normal breast-like, HER-2 overexpressing, Luminal A, Luminal B subtypes was 58.94% (56/95), 39.39% (65/165), 33.33% (22/66), 17.91% (79/441) and 5.56% (2/36), respectively. The positive expression rate of PLK1 in ER-negative breast carcinomas (basal-like, normal breast-like and HER-2 over-expressing subtype) was significantly higher than that in ER-positive subtypes (luminal A and lumina B) (P<0.05). The expression of PLK1 was negatively correlated with ER and positively correlated with Ki-67 (P<0.01), while there was no significant correlation with the expression of PLK1 and HER-2. In cases of ER negative breast cancer, the expression of PLK1 in basal-like breast subtype was much higher than that in normal breast-like and HER-2 over-expressing subtype. The expression of PLK1 was correlated with lymph node metastasis and pTNM stage (P<0.05) of basal-like breast cancer. Conclusion: The overexpression of PLK1 may be closely correlated with ER-negative breast cancer, especially with basal-like breast cancer, and PLK1 may play an important role in the invasion and metastasis of basal-like breast carcinoma.