Angiogenesis is crucial for solid tumor growth. By secreting angiogenic factors, tumor cells induce angiogenesis. However, targeting these angiogenic factors for cancer therapy is not always successful, suggesting that other factors may be involved in tumor angiogenesis. This work shows that 25 protein spots were differentially expressed by two-dimensional gel electrophoretic analysis when HepG2 cells induced endothelial cell differentiation to tube in vitro, and most of them were upregulated. Twenty-one proteins were identified with MALDI-TOF-MS, and the other four were identified by LTQ-MS/MS. Keratins were identified as one class of these upregulated proteins. Further study indicated that the expression of keratin 17 in cultured endothelial cells is likely microenvironment regulated, because its expression can be induced by HepG2 cells and bFGF as well as serum in culture media. Increased expression of keratins in endothelial cells, such as keratin 17, may contribute to the angiogenesis induced by HepG2 cells.