Disclosed is a therapeutic method for inhibiting development or growth of tumors that are resistant to the blockade of delta-like ligand 4 (Dll4), or vascular endothelial growth factor (VEGF), or to other therapeutic agents, by administering the combination of Dll4 antagonist and VEGF antagonist. The combined administration of these two agents, concurrently or sequentially, exhibits synergistic effects on blood vessel development and growth, thereby more effectively inhibiting the tumor growth than an administration of either agent alone. The Dll4 antagonist can be an anti-Dll4 antibody or antibody fragment capable of inhibiting the binding of Dll4 to a Notch receptor, or a fusion protein comprising the extracellular domain of Dll4 or a soluble Notch receptor, or a fragment thereof, fused to a multimerizing component. The VEGF antagonist can be a VEGF trap, anti-VEGF antibody or antibody fragment capable of inhibiting the binding of VEGF to a VEGF receptor.