Tumor growth and metastasis are dependent on angiogenesis. Tumor endothelial cells (TECs) lining tumor blood vessels, are important targets in cancer therapy. TECs have traditionally been considered to be the same as normal ones. However, tumor blood vessels have a distinctively abnormal phenotype, including morphological alterations. Recently, it has been revealed that TECs constitute a heterogeneous population, exhibiting characteristics that are induced by tumor microenvironmental factors. Furthermore, TECs contribute to cancer progression through metastasis. For example, TECs in highly metastatic tumors aberrantly express angiocrine factors which stimulates cancer cell intravasation, in turn they instigate tumor cells to metastasize. Recently, we found biglycan induced tumor fibrosis which is barrier for immune cells. Biglycan knockout mouse tumor showed more CD8 cells, suggesting tumor immunity is activated by biglycan inhibition. Besides, we have found that TECs express one of drug transporter, ABCB1, and support cancer cells even during chemotherapy. Targeting such abnormal TECs could show the anti-tumor effects in the mouse model. TEC abnormalities related to cancer progression will be to provide insight into new anticancer therapies.