Adult hematopoietic stem cells (HSCs) are quiescent most of the time, and how HSCs switch from quiescence to proliferation following hematopoietic stress is unclear. Here we demonstrate that upon stress the coxsackievirus and adenovirus receptor CAR (also known as CXADR) is upregulated in HSCs and critical for HSC entry into the cell cycle. WT HSCs were detected more rapid repopulation ability than the CAR cKO counterparts. After 5-FU treatment, CAR cKO HSCs had lower levels of Notch1 expression and elevated protein level of Numb, a Notch antagonist. The Notch signaling inhibitor DAPT, dominant negative form of MAML (a transcriptional coactivator of Notch), or dominant negative mutant of LNX2 (an E3 ligase that acts on Numb and binds to CAR), all were capable of abrogating the function of CAR in HSCs. We conclude that CAR activates Notch1 signaling by downregulating Numb protein expression to facilitate entry of quiescent HSCs into the cell cycle during regeneration.