MicroRNAs are endogenously expressed small noncoding RNAs that regulate gene expression on the posttranscriptional level.Previous works indicated that miR-17-92 cluster could regulate endothelial cells (Ecs) functions involved in angiogenesis.We have identified microRNA-17-3p (miR-17-3p), a component ofmiR-17-92 cluster, could control the angiogenic activity of human umbilical vein endothelial cells (HUVECs) in a cellautonomous manner in vitro.A 21-bp fragment from Flk-1 3-untranslated region (3-UTR) containing miR-17-3p targeting sites was required for the rapid down-regulation of Flk-1 expression by in silico and experimental analysis.Subsequently, the downstream of cell growth pathway was inhibited by forced up-regulation of miR17-3p.Based on these data, we conclude that miR-17-3p is a negative regulator of angiogenic phenotype of Ecs through its ability to modulate the expression of Flk-1, which is implicated the pleiotropie effects of miR-17-92 in angiogenesis.