Apoptosis of the cell is one of the key steps in tissue remodeling and functional differentiation. At the time of implantation there are dramatically remodeling and functional differentiations of uterine endometrium. Decidualization is the process by which the uterine stromal cells proliferate and differentiation into morphologically and functionally distinct decidual cells. Bax is one of the key molecules in apoptotic process during that time. It is known that Bax expression have characters of time specificity and tissue specificity but the role of implantation is not largely unknown. The aim of this study was to examine the embryo implantation and decidualization reaction in order to understand the role of pro-apoptotic Bax gene as a major regulator of apoptosis of decidual cells. Some of Bax knockout mice have coat pigmentation defects involving white color. The number of blue bands after embryo transfer (ET) was not different between the control and Bax-knockout (Bax-KO) black coat (BC) and white coat (WC). Decidua were normally differentiated in both control and Bax-KO BC, but deciuda were not detected in Bax-KO WC at 96 hr post ET. Moreover, embryos of Bax-KO WC stayed in dormant stage on the embryonic day 7.5 and day 9.5. These results suggest that Bax and its related unknown genes may be a key molecule in implantation and uterine decidual reaction.