Objective: The purpose of this study is to investigate the effect of cumulus cells on maturation,fertilization and subsequent development of mouse germinal vesicle oocytes.Materials and Methods: A total of 470 germinal vesicle (GV) oocytes were obtained from26 ovaries of 3- 4 week old ICR female mice 48 hours after injection of 5 IU pregnant mareserum gonadotropin (PMSG). Collected oocytes were divided into two groups; group I: GVoocytes without cumulus cells (denuded oocyte), group II: GV oocytes with cumulus cells(cumulus-oocyte complex). The oocytes in both groups were cultured in TCM-199 mediumsupplemented with 10% fetal bovine serum (FBS) for 22- 24 hours in a humidified atmosphereof 5% CO2 in air at 37°C. Oocyte maturation was scored under inverted microscope.To do in vitro fertilization, matured oocytes from each group were placed in T6 mediumand capacitated spermatozoa were added. Then the fertilized oocytes were cultured andassessed for cleavage to the 2-cell stage 24 hours and production of blastocyst 120 hoursafter fertilization. Data was analyzed by chi-square test and differences in the values wereconsiderable significant when p<0.05.Results: Maturation, fertilization, cleavage and blastocyst rates in denuded oocytes were:76.32%, 57.49%, 51.15% and 19.14% respectively. In the cumulus-oocyte complex rateswere: 89.41%, 80.76%, 75.58% and 45.62% respectively; all in the cumulus-oocyte complexwere significantly higher than those of denuded oocytes (p<0.05).Conclusion: The present study indicates that cumulus cells have important role duringmaturation, fertilization and subsequent embryo development to the blastocyst stage.