Introduction: Antiangiogenic therapy has been thought to hold significant potential for the treatment of cancer. Inhibition of vascular endothelial growth factor (VEGF) pathway impedes tumor growth and, clinically, the VEGF-neutralizing antibody has been used as anticancer treatments in several tumor types including colorectal cancer. However, several studies indicate antiangiogenic therapy may have limited clinical benefit. Here we investigated the alteration of colorectal cancer phenotype in vivo during antiangiogenic therapy. Methods: TK-4, a solid tumor strain, derived from human colon cancer was utilized. TK-4 was implanted orthotopically into cecal walls of BALB/c nu/nu mice. The animals were treated with anti-human VEGF monoclonal antibody (treated group, n=14, 10μg/body, i.p, thrice a week) or control IgG (Control group, n=14), The animals were sacrificed and tumors were removed at two time points, day21 (n=4, each group) and day42 (n=10, each group). The tumors removed at day42 were examined for tumor volume and microvessel density (MVD) using CD31 immunohistochemistry and gene expression was analyzed using microarrays (Human Gene 1.0ST Array, Affymetrix). Mitosis and apoptosis were evaluated using H&E staining and TUNEL assay, respectively in the tumors at day21 and day42. Hypoxia in tumors was evaluated by HIF-1α immunohistochemistry. Results: Tumor volume and MVD were significantly lower in treated group than those in control (p=0.013, p=0.003, respectively). Mitosis was significantly lower in treated tumors than control at day21 (p=0.049), but not at day42. Whereas TUNEL positivity was comparable between the two groups at day21, it was lower in treated tumors at day42 (p=0.003). Gene set enrichment analysis revealed that the gene set “HYPOXIA_REG_UP” was upregulated in treated group. HIF-1α positivity was significantly higher in treated tumors (p=0.010). Conclusion: Tumor hypoxia appears to play important roles in enhancing or selecting tumor cells with high malignant potential.