Endothelial cell proliferation supporting angiogenesis requires sufficient nutrient supply because of facilitated intracellular metabolism. However, little is known about the mechanism for the promotion of nutrient incorporation in proliferating endothelial cells. Here we show that L-type amino acid transporter 1 (LAT1) is a major transporter of essential amino acids in human umbilical vein endothelial cells (HUVECs). Growing HUVECs express a certain level of LAT1. A LAT1-specific inhibitor suppressed leucine uptake, cell proliferation, and tube formation of HUVECs. Therefore, LAT1 acts to support effective uptake of amino acids, which is critical for the optimal function of HUVECs for angiogenesis.