Tumor angiogenesis is a mandatory step of tumor growth, invasion, and metastasis involving endothelial cell proliferation and migration from blood vessels in peritumoral tissues and formation of tubules. Water channel aquaporin-1 (AQP1) has been reported to express in microvessels of many different type of tumors, indicating the possible involvement of AQP1 in tumor angiogenesis. A melanoma-bearing model in AQP1-knockout mice was used to evaluate the role of AQP1 in tumor angiogenesis. The results demonstrated nearly 30% retarded growth of subcutaneously inoculated melanoma in AQP1 knockout mice compared to tumors in wildtype mice. Immunohistochemistry of melanoma sections revealed strong AQP1 protein labeling in endothelium of tumor blood vessels in wildtype mice and negative labeling of AQP1 in the counterpart structures in AQP1-knockout mice. On the same tumor sections stained by hematoxylin, melanoma cells form islands with microvessels in the center. It is easily seen that high-density and thin microvessels (142/mm^{2) locate in the center of tumor islands in wildtype mice, whereas sparse and enlarged vessels (47/mm2) are evident in tumor islands in AQP1 knockout mice coincident with larger necrotic area in outer layer of the islands. These results provide definitive evidence that lacking of water channel AQP1 results in defective tumor angiogenesis and retarded tumor growth that may be involved in insufficient blood supply and/or abnormal transendothelial fluid transport. The underlying molecular and cellular mechanisms are worth further investigation.}