Abstract: Objective Immature and mature mouse models were used and the effect of age on mouse EM>in vitro/EM> fertilization were identified in order to make full use of high quality animal resources, to shorten the intervals between generations and efficiently to improve the embryo engineering technology. Methods The ovulation rate, the in vitro fertilization rate and the blastocyst rate were measured after pregnant mare serum gonadotropin（PMSG）/ human chorionic gonadotrophin( hCG ) superovulation of both the immature and mature mice as well as the ovarian morphological changes were observed. At the same time, the role of genetic factor in the mouse in vitro fertilization was evaluated. Results The ovulation rate in immature mice was significantly higher than that in mature mice, and antral follicles in immature mouse ovaries were more than in mature mouse ovaries. There was a significantly lower fertilization rate from immature mouse oocytes than from mature mouse oocytes, however, there was no remarkable difference on the blastocyst rate between these two groups. Further analysis of usable oocytes in the mature mouse group found that the high response group to superovulation (usable oocytes ≥40) had significantly higher fertilization rate and blastocyst development rate than those of the low response group to superovulation (usable oocytes 40). In addition, the ovulation rate and the follicle development among different individuals in the same litter were similar. Conclusion There is a high response to superovulation in the immature mice, while in the mature group, the ovarian response to superovulation

Key words: Ovary, Oocyte, Superovulation, EM>In vitro/EM> Fertilization, Mouse