An HGF antagonist, NK4, inhibits not only invasion and metastasis of tumor cells driven by HGF-Met receptor binding, but also tumor angiogenesis. To address the antitumor activities of NK4, we investigated the biological behaviors of CT26 transfected with the NK4 gene (CT26-NK4) in vitro and in vivo. In the in vitro assay, the invasion in MOCK transfected cells (control) was stimulated by HGF; however, in CT26-NK4 cells, these effects were completely inhibited. In the in vivo assay, the tumor growth of CT26-NK4 was strongly suppressed and the survival of CT26-NK4 tumor-bearing mice was significantly prolonged. Immunohistochemical analysis revealed that while proliferating cells (PCNA immunostaining) of CT26-NK4 tumors were weakly suppressed, the micro-vessel number (CD31/PECAM-1 immunostaining) in those tumors was significantly suppressed as compared with the control tumors. In conclusion, NK4 exerts potent antitumor effects via anti-angiogenesis rather than inhibition of biological events of tumor cells stimulated by HGF.