In order to investigate the development of Kunming mouse preimplantation embryos cultured in vitro under simulated microgravtiy, one-cell and 4-cell embryos of Kunming mouse (Fertilization In Situ) were cultured in CZB or KSOM media under simulated microgravity or normal gravity environment respectively. The results showed that under normal gravity, the percentage of passing 2-cell development block embryos was not different in CZB with in KSOM, but the percentage of blastocysts was lower in CZB than that in KSOM significantly. The percentages of passing 2-cell development block embryos and blastocysts in CZB or KSOM were lower under simulated microgravity than those under normal gravity. It is suggested that the frequency of early embryonic lethality is possibly increased by simulated microgravity.