Abstract: In order to investigate the effects of c-Type Natriuretic Peptide（CNP） on the transition from 2-cell to 4-cell and blastocyst cavity formation in mice, the mouse prokaryotic embryos were collected and divided into 3 groups for in vitro culture, which were control group, group 1 and group 2. The doses of 0, 100 μg/L and 200 μg/L of CNP were added to control group, group 1 and group 2, respectively, at different developmental stages of embryos. When 2-cell embryos were developed, adding CNP to the culture mediums for observation of the 4-cell rate and blastocyst rate. When compacted morula were formed, adding CNP to the culture mediums for observation of blastocyst cavity formation rate, blastocyst expansion rate and blastocyst hatching rate, as well as the diameter of expanded blastocyst, the ratio of blastocyst area and the number of blastocyst cells. The results suggested:（1） With the addition of CNP at the 2-cell stage, the 4-cell rate and blastocyst rate of the group 2 were significantly reduced compared with the control group（P＜0.05）.（2） The addition of CNP at the stage of compacted morula did not affect the blastocyst cavity formation rate, the expansion rate of blastocyst, the diameter of blastocyst, the ratio of blastocyst area and the hatching rate in group 1（P＞0.05）. However, the CNP addition significantly decreased the total cell number of blastocysts, compared with the control group（P＜0.05）.（3） CNP addition at the stage of compacted morula significantly reduced the blastocyst cavity formation rate, the rate of blastocyst expansion, the hatching rate, the diameter of blastocyst, the ratio of blastocyst area and the number of blastocyst cells in group 2, compared with the control group（P＜0.05）. It can be deduced that exogenous addition of CNP could inhibit the transition of embryos from 2-cell to 4-cell in mice, blastocyst cavity formation and expansion, and reduced blastocyst cell number.