Early differentiation of vascular endothelial cells is indispensable for the proper patterning and lumen formation of blood vessels. The vascular network forms in a reproducible and evolutionarily conserved manner, and their molecular mechanisms of formation have been markedly focused on. However, the differentiation process of vascular progenitors, angioblasts, remains unclear. Here, we report several tools to detect and analyze undifferentiated vascular endothelial cells in zebra sh embryos, including the vascular-speci c transgenic lines, inhibi- tor treatments, and 3D reconstruction technique of electron microscopic serial images.