Notch signaling is essential for the self-renewal of mammalian neural progenitor cells. A variety of mechanisms modulate Notch signaling to balance the self-renewal and differentiation of progenitor cells. Fringe is a major Notch regulator and promotes or suppresses Notch signaling, depending on the Notch ligands. In the developing brain, Lunatic fringe (Lfng) is expressed in self-renewing progenitors, but its roles are unknown. In this study, in vivo mosaic analyses using in utero electroporation were developed to investigate the roles of Lfng in neural progenitor cells. We found that Lfng potentiates Notch signaling cell-autonomously. Its depletion did not affect the balance between neuronally committed cells and self-renewing progenitors, however, irrespective of the cell density of Lfng-depleted cells, and caused no obvious defects in brain development. In vivo overexpression experiments with Notch ligands suggest that Lfng strongly augments Notch signaling mediated by Delta-like 1 but not Jagged 1.