Objective To explore the relationship between the neocortical lamination and cell cycle, especially the cell cycle alteration of postmitotic neurons in their migration, proliferation, and the cortical development. Methods BR>A total of 200 mice were used in this study. The immunofluorescent staining, DiI tracing and 5-bromodeoxyuridine (BrdU) assay were used to observe changes in the cerebral cortex of embryonic and postnatal mice. Densities of BrdU and Cyclin D1 positive cells of the cerebral cortex were measured. Results At postnatal day 0 (P0), the lamination of deep cortical layers (Ⅵ-Ⅴ) started. The trend of lamination of superficial cortical layers (Ⅳ-Ⅱ) began after P5. The six layers were fully formed at P7.At P14, the feature of lamination in the cerebral cortex was fully formed, and then stabilized at P30. In the process of lamination, pyramidal cells were oval-shaped and had small branches with their dendritics at embryonic day 17 (E17). At P15, the pyramidal cells were mature and appeared with complex apical dendrites and base dendrites. With BrdU assay, we found that there were many proliferative stem cells in ventricular zone and subventricular zone. In the meantime, the postmitotic neurons that were proliferated from BrdU positive progenitor cells migrated into parenchyma of the neocortex. Statistical analysis showed that migrating postmitotic neurons into cortical plate(CP) decreased gradually from P0 to P30.The cell cycle of the migrating neurons was analyzed with GSUB>1/SUB> phase specific marker, Cyclin D1.The study showed that migrating neurons were in GSUB>1/SUB> phase after mitotic division, and they exited cell cycle once settlement in CP. The number of Cyclin D1 positive cells in the neocortex changed with parabolic curve and peaked at P12. After P30, only a few cyclinD1 positive cells were found in CP. Conclusion The neuroproliferation, cell differentiation and neuron migration are involved in neocortical lamination, which company with the maturation of pyramidal cells in the cortical plate. The neuroproliferation and neuron migration mainly occur in embryonic and neonatal periods. The migrating postmitotic neurons are in GSUB>1 /SUB>phase. The differentiation of migrating postmitotic neurons is the transition from GSUB>1 /SUB>to GSUB>0/SUB>.Once they settle in CP, the postmitotic n