Abstract This study was to investigate the expression of Cdk inhibitors p27 kip1 and p57 kip2 during the development of mouse placenta and during the steroid-treated culture of human choriocarcinoma JEG-3 cells. The p27 kip1 mRNA in mouse placenta was highly expressed in 18 days p.c. than that in other groups. But, p57 kip2 mRNA expression was high in 12, 14, and 16 days p.c., then decreased in 18 days p.c. The p27 kip1 protein expression pattern was similar to mRNA. But, p57 kip2 expression was higher in 14 days p.c. than that in other groups. The p27 kip1 protein in mouse placenta was gradually increased in labyrinth zone from 12 days to 18 days p.c. However, p57 kip2 protein was slightly decreased in labyrinth zone from 12 days to 18 days p.c. These reverse patterns of p27 kip1 and p57 kip2 expression were also shown in decidua and spongiotrophoblast. The p27 kip1 mRNA expression was very low in human choriocarcinoma JEG-3 cells with estradiol concentration-independent manner. In 5 and 50 ng DEX-treated groups, p27 kip1 mRNA was dramatically increased in comparison with control groups. The p57 kip2 mRNA was not detected in JEG-3 cells. This result shows that p27 kip1 may play a role in late period of mouse placental development and p57 kip2 may play a role in middle period of mouse placental development, and that p27 kip1 may play a role in growth inhibition of human choriocarcinoma cells and could be up-regulated by DEX in human choriocarcinoma.