Melittin (1) is a major polypeptide in honey bee venom that has been used traditionally against chronic inflammation and cancer. However, its molecular mechanism has not been determined. In this study, the antitumor effect of 1 was compared with that of NS398, a cyclooxygenase-2 (COX-2) inhibitor, in vivo and in vitro. Subcutaneous injection of 1 at 0.5 and 5 mg/kg suppressed significantly vascular endothelial growth factor (VEGF)-A-transfected highly metastatic Lewis lung cancer (VEGF-A-hm LLC) tumor growth by 25% and 57%, respectively. Also, 1 inhibited significantly the number of vessels around VEGF-A-hm LLC cells. The results were superior to those obtained in the mice treated with NS398. Compound 1 dose-dependently inhibited proliferation and tube formation in human umbilical vein endothelial cells (VEGF-A-HUVECs), without affecting cell viability in native HUVECs. In addition, 1 decreased the expression of VEGF receptor-2 (VEGFR-2), COX-2, and prostaglandin E2 (PGE2) in VEGF-A-transfected HUVECs. These effects were accompanied by a reduction of the phosphorylation of extracellular signal-regulated kinase 1/2 and c-jun N-terminal kinase, whereas it increased the phosphorylation of p38 mitogen-activated protein kinase (MAPK). SB203580 abolished the downregulation of COX-2 and VEGFR-2 and the inhibition of cell proliferation by 1. The antitumor activity of 1 may be associated with antiangiogenic actions via inhibiting VEGFR-2 and inflammatory mediators involved in the MAPK signaling pathway.