Objective To study the effects of different contents of brain-derived neurotrophic factor(BDNF) on the expression of cell cycle\|related proteins in G2 phase in differentiated SH\|SY5Y cells in vitro. Methods Sequential exposure of SH\|SY5Y cells to retinoic acid(RA) for 5 days and various contents of brain\|derived neurotrophic factor(BDNF 1μg/L, 10μg/L, 100μg/L) for 2 days yielded homogeneous populations of cells with neuronal morphology. At the same time, control group was incubated in insulin-transferrin selenium-X(ITS) alone for 7 days; RA group was incubated in RA only for 7 days. All media were prepared with DMEM\|F12 containing 1% ITS without serum. Real time one\|step RT\|PCR was carried out to determine the expresson of cell cycle\|related protein in G2 phase. Fluorescence activated cell sorting (FACS) was used to analyze the phase of cell cycle. Results The results were shown that the expression of cyclin B1 was downregulated significantly comparing to RA group and ITS group (P<0.05) in all BDNF groups; The expressions of cyclin B2 and Cdk1 were decreased only in 100μg/L BDNF group comparing to ITS group and RA group(P<0.05)； No change of the expression of Cdk5 was found in all BDNF groups comparing to ITS group. The cell percent of G1 phase in all BDNF groups was increased significantly comparing to RA group and ITS group(P<0.01); the cell percent of S phase was decreased significantly comparing to RA group (P<0.01) in all BDNF groups, and decreased significantly comparing to ITS group in 1μg/L BDNF group and 100μg/L BDNF group(P<0.01).Conclusion The results suggest that BDNF doesn’t enhance the mRNA level of cell cycle\|related proteins of G2 phase, combining the change of cell percent in G1 and S phase certifies that