Introduction: Endothelial cell (EC) dysfunction is a hallmark of vascular aging, contributing to age-related vascular diseases. Elucidating EC aging at the single cell level has significant scientific and therapeutic potential.Research Question: What is the landscape of EC aging at the single cell level?Methods: Mouse aortas were harvested from 1-, 10-, and 20 month-old mice (n=2 males and 2 females at each time point). Pooled single-cell suspensions were prepared, and PECAM1+ cells were enriched by magnetic-activated cell sorting.Results: Single cell RNA sequencing revealed 879 ECs, categorized into six subpopulations (EC 1-6). Of the three canonical EC markers (Pecam1, Cdh5, and Kdr) EC1 and 2 showed higher expression of Pecam1, whereas EC3 and 4 showed higher expression of Cdh5 and Kdr. ECs from young (1 month-old), middle-aged (10 month-old), and elderly mice (20 month-old) were enriched in EC1, EC3, and EC4, respectively. Gene regulatory network analysis identified specific regulons for each EC subcluster. EC1 displayed progenitor-like features (Isl1 as one of the top regulons, Procr and Cd34 as highly expressed genes) with high angiogenic potential (Twist1 as one of the top regulons, Timp2, Mmp14, and Mmp2 as highly expressed genes) and the longest G1 phase suggesting its quiescence. EC3 cells exhibited high proliferative status demonstrated by G2M or S phase, accompanied by upregulation of stress-responsive TF Atf3 and NF-κB family members. EC4 demonstrated pronounced Pparg regulon activity. Genes directly regulated by Pparg (Scarb1 and Cd36), and genes involved in fatty acid metabolism and atherogenesis (Tcf15, Fabp4, Gpihbp1, Lpl, and Meox2) were highly expressed in EC4 and EC3.Conclusions: Our analysis identified four distinct EC subpopulations through the aging process of mouse aortic ECs: EC1, Pecam1 progenitor-like EC; EC2, Pecam1 common EC; EC3, Cdh5 Kdr inflammatory atherogenic EC; and EC4, Cdh5 Kdr non-inflammatory atherogenic EC.