Leukemia inhibitory factor (LIF) is a 45-56 kDa glycoprotein that has an important role in proliferation and embryo implantation. Its effect on oocyte maturation and how to exert the function remained to be elucidated.Immature mice superovulated with human menopausal gonadotropin and germinal vesicle (GV) oocytes were obtained from ovary 48 hours after. GV oocytes were cultured in tissue culture medium 199 with 0, 100, 500 and 1,000 U/ml LIF. Cumulus expansion and in vitro maturation (IVM) rate were assessed after culture. For reverse transcriptase PCR, total RNA from GV and metaphase II (MII) oocytes were extracted by Trizol reagent. The quantity and quality of RNA were determined by spectrophotometry and electrophoresis, respectively. Reverse transcription was performed by Super Script III reverse transcriptase with 1 µg of total RNA followed by DNase I treatment and heat inactivation. Expression of gp130 was determined by RT-PCR.Our results showed that cumulus expansion was improved with 1,000 U/ml in culture medium compared to others. GV breakdown and MII rate in groups with LIF were higher than control group and were dose-dependent. In 1,000 U/ml, LIF rate of MII was significantly higher than control group (P less than 0.05). Our results also showed that gp130 is expressed neither in GV nor in MII oocytes during IVM of mouse oocytes.gp130 is expressed in human oocyte but not in mouse. Our results suggest that in mouse, LIF could affect the oocyte via another receptor or via cumulus cells; however, further studies are warranted.