Abstract: This study was conducted to explore the mRNA expression of cathepsin (CTSB, CTSS) in cumulus cells during in vitro maturation(IVM) from sheep and to detect the correlation beween the mRNA expression of CTSB and CTSS in cumulus cells and in vitro developmental capacity of oocytes. The expression abundance of CTSB and CTSS in cumulus cells of sheep was detected at IVM 0 h，IVM 8 h，IVM 16 h and IVM 24 h through Realtime PCR. The expression abundance of CTSB and CTSS in cumulus cells was detected before and after IVM in the mature medium which contains cathepsin inhibitor E64 with different concentration. The effect of E64 with different concentration in mature medium on in vitro maturation, fertilization and developmental capacity of oocytes were also observed. The mRNA expression of CTSB and CTSS in cumulus cells was downregulated continuously during in vitro maturation (IVM 0 h, IVM 8 h and IVM 16 h, P<0.01), the mRNA expression of CTSB and CTSS in the groups with 5 and 10 μmol·L-1 E64 for IVM 24 h were significantly lower than that of the control and 1 μmol·L-1 E64 groups, nevertheless, there were no significant difference between the two former groups (P>0.05). The blastocysts rate in the groups of 5 and 10 μmol·L-1 E64（26.82%, 30.95%) were higher than those of the control and 1 μmol·L-1 E64 groups(15.63%, 18.23%, P<0.05), but there were no significant difference in the percentage of oocytes mature and cleavage rate among all the groups (P>0.05). The result demonstrate that the mRNA expression abundance of CTSB and CTSS in cumulus cells from sheep is downregulated continuously during IVM, there is negative correlation between the mRNA expression of CTSB and CTSS in cumulus cells and the following embryo developmental capacity；the cathepsin inhibitor E64 could improve the capacity of oocytes developing into blastocysts after fertilization by downregulating the mRNA expression abundance of CTSB and CTSS in cumulus cells.