Various adjuvants have been tested clinically for patients with problems with embryo implantation during in vitro fertilization (IVF)-embryo transfer (ET). Vitamin D3, an essential modulator of various physiological processes, has received attention as an important adjuvant for successful pregnancy, as many studies have shown a strong association between vitamin D deficiency and implantation failure and fetal growth restriction. However, vitamin D has been widely utilized in different protocols, resulting in non-reproducible and debatable outcomes. In the present study, we demonstrated that cyclic intrauterine administration of vitamin D3 increased endometrial receptivity and angiogenesis, which could be attributed to increased recruitment of uterus-resident natural killer cells. In particular, cyclic treatment of vitamin D3 promoted stable attachment of the embryo onto endometrial cells in vitro, suggesting its merit during the early stage of embryo implantation to support the initial maternal-fetal interactions. Our findings suggest that women with repeated implantation failure may benefit from the use of vitamin D3 as a risk-free adjuvant prior to IVF-ET procedures to improve the uterine environment, and make it favorable for embryo implantation.