It is known that the liver undergoes size increase and differentiation simultaneously during the postnatal period. Cells in the liver undergo a period of well-controlled proliferation to achieve the adult liver-to-body weight ratio. The postnatal liver growth is also accompanied by simultaneous hepatic differentiation. However, the mechanisms of liver size regulation and differentiation are not completely clear. Herein we report that yes-associated protein (Yap), the downstream effector of the Hippo Kinase signaling pathway, plays a role in liver size regulation and differentiation during the postnatal liver growth period. Postnatal liver growth was studied in C57BL/6 mice over a time course of postnatal days (PND) 0-30. Analysis of nuclear Yap by Western blot indicated peak Yap activation between PND15-20, which coincided with increased cyclin D1 expression and liver cell proliferation. Analysis of postnatal liver development in Yap(+/-) mice revealed a significant decrease in the liver-to-body weight ratio compared with Yap(+/+) mice at PND15 and -30. Yap(+/-) mice exhibited a significant decrease in postnatal liver cell proliferation, but no change in apoptosis was observed. Furthermore, global gene expression analysis of Yap(+/-) livers revealed a role of Yap in regulation of genes involved in bile acid metabolism, retinoic acid metabolism, ion transport, and extracellular matrix proteins. Taken together, these data indicate that Yap plays a role in both cell proliferation and possibly in hepatic differentiation during postnatal liver development.