Mice preantral follicles were cultured in vitro for 12 days to achieve metaphase Ⅱ (MⅡ) oocytes. Oocyte growth differentiation factor-9 (GDF-9) gene expression was measured during different growth stages to explore the relationship between oocyte maturation and GDF-9 gene expression. Preantral follicles of 10-day old mice were isolated from the ovaries and were cultured for 12 days. Oocytes from day 2 (D2), D4, D6, D8, D10, D12 cultured in vitro were named the in vitro group and oocytes of day 12 (D12), D14, D16, D18, D20, D22 grown in vivo were named the in vivo group. Follicle survival, antrum formation and maturation rate were 89.5%, 51.8% and 56.6% respectively in follicles cultured in vitro. After RT-PCR and agarose gel electrophoresis, relative mRNA abundance of GDF-9 was measured in each group of oocytes. At day 8-12, the GDF-9 gene expression level of oocytes in vitro was significantly lower than that in vivo (P<0.05). We conclude that MⅡ oocytes can be obtained from in vitro culture of preantral follicles. The GDF-9 gene expression of oocytes varies at different growth stages in vivo. The low expression of GDF-9 in oocytes cultured in vitro may be the cause of their low developmental capacity.