Aim: To investigate whether human neural progenitor cells (hNPCs) could be induced to differentiate toward a neuronal phenotype by all-trans Retinoic acid (RA), and to study whether p27 Kip1 played a key role in the differentiation process of hNPCs. Methods: hNPCs were derived from the striatums of human embryos at 12 weeks' gestation and cultured with serum-free medium in presence of EGF and bFGF. At the appropriated time, hNPCs were exposed to 1 μmol · L -1 RA for 3, 5, 7 days respectively. The properties of hNPCs were characterized by using flow cytometry analysis (FACS) and Western blot. Results: Cell cycle analysis was performed by FACS. Typically, 65.20% of proliferating hNPCs were in the G 1 /G 0 -phase while 28.39% of cells in the S-phase. Following RA treatment, cell growth was arrested, and 85.72% of cells accumulated in G 1 /G 0 - phase while 8.62% of cells in the S-phase. Western blot analysis demonstrated that the levels of p27 Kip1 in the hNPCs increased following 3 days' treatment with RA compared with those of untreated cells, with a peak at 5th days (P <0.05). The same results were acquired both in nuclear proteins and in cytoplasm proteins. The expression level of (s-phase kinase-associated protein 2, skp2) decreased significantly in response to RA treatment. RA did not affect the expression of cdk2, but the expression of p-cdk2, which represented the activity of cdk2, was markedly decreased in response to RA treatment. Proliferating cell nuclear antigen (PCNA) could be detected only in normal untreated cells and disappeared when cells were induced to differentiate by RA. skp2, which was required for the ubiquitin-mediated degradation of p27 Kip1 , was detected in proliferating hNPCs. The expression of skp2 reduced dramatically in response to RA treatment in a time-dependent manner. Conclusions: There was a functional link between RA and p27 Kip1 function in the control of neuronal differentiation in hNPCs. p27 Kip1 played a key role in neuronal differentiation. Moreover, high levels of p27 Kip1 were regulated via inhibiting its degradation through reducing proteasome-dependent proteolysis.