Objective To investigate the clinicopathologic and immunohistochemical characteristics of breast carcinosarcoma with osteosarcoma components and its differential diagnosis. Methods The pathologic features and clinical manifestations of the two patients were retrospectively reviewed. Immunohistochemistry was performed and the literature was reviewed. Results Histopathologically, the neoplasm consisted of invasive ductal carcinoma of no special type and poorly ( case 1 ) or well (case 2) differentiated osteosarcomatoid elements. The morphological transition from carcinoma to sarcomatoid elements was seen. Immunohistochemically,the carcinoma cells were positive for CK and EMA , the sarcomatoid elements were stained positive for vimentin, and a few cells of two elements were positive for S-100 protein. Ki-67 and VEGF were over expressed in both elements of Case 1.In case2 Ki-67 expression was low in both elements and VEGF over expression was only seen in sarcomatoid elements. Some of the carcinoma cells were positive for ER. Conclusions Carcinosarcoma of the breast with osteosarcomatoid elements is a rare type of mixed epithelial/mesenchymal metaplastic breast carcinoma. The types and proportion of carcinoma and sarcomatoid elements determine the diagnosis.