Abstract: Objective To explore the relation between oocyte maturation and growth differentiation factor-9 (GDF-9) gene expression. Methods Ovariectomy was performed in 50 Kunming female mice of 10 days old, and the preantral follicles were isolated from the ovaries and cultured in medium drops for 12 days. Oocytes and somatic cells were mechanically isolated. The oocytes cultured in vitro for 2, 4, 6, 8, 10, and 12 days constituted the in vitro cultured group and the oocytes obtained from female mice of 12, 14, 16, 18, 20, and 22 days old served as the in vivo group. Semi-quantitative RT-PCR and agar gel electrophoresis were performed to quantify GDF-9 gene expression in each oocyte. Results Follicle survival, antrum formation and maturation rate was 89.5%, 51.8% and 56.6% in the in vitro cultured follicles, respectively. GDF-9 gene expression on days 2, 4, 6, 8, 10, and 12 in in vitro cultured oocytes was 0.83±0.08, 0.52±0.09, 0.45±0.13, 0.49±0.09, 0.49±0.09, and 0.68±0.08, respectively; GDF-9 gene expression in in vivo grown oocytes of 12, 14, 16, 18, 20, and 22 days were 0.64±0.35, 0.48±0.10, 0.52±0.10, 0.66±0.08, 0.72±0.09, and 0.91±0.11, respectively. Between days 8 and 12, GDF-9 gene expression in in vitro cultured oocyte was significantly lower than that in in vivo grown oocytes (P<0.05). Conclusion MII oocytes can be obtained from in vitro culture of the preantral follicles. GDF-9 gene expression in the oocytes varies with their growth stages. Between days 8 and 12 of in vitro culture, GDF-9 gene expression in the cultured oocytes is different from that in in vivo grown oocytes.