NIRF is a RING finger protein with a ubiquitin-like domain, a PHD finger, a YDG/SRA domain, and a RING finger domain. Previous study showed that NIRF is a nuclear protein expressed in association with cell proliferation. In this study, we further characterized NIRF functions in cell cycle regulation. Flow cytometric analysis showed that overexpression of NIRF induced an increase in G1 phase cells. Immunoprecipitation and immunoblotting experiments showed that NIRF bound to the inactive Cdk2-cyclin E complex. There existed phosphorylated NIRF in cells, and dephosphorylated NIRF interacted with Cdk2. NIRF was phosphorylated by Cdk2 in vitro. These results suggest that NIRF may participate in the G1/S transition regulation.