Natural killer (NK) cells are bone marrow-derived lymphocytes. They produce cytokines that regulate the development of acquired immunity. In view of their accumulation at the maternal-fetal interface, uterine natural killer (uNK) cells are also thought to play essential roles during pregnancy. Our results compared the differences of cytokine secretion profile by NK cells in uterine endometrium, liver, spleen and peripheral blood, and focused on the cytokines secretion by uNK cells. It was demonstrated that the expression of IFN-γ and TNF-α in uterine endometrium of pregnant mice are lower than those in liver, but they increase significantly during pregnancy. Our study showed that the number of uNK cells was increased significantly during pregnancy. They produced more IFN-γ and TNF-α than other organ-derived NK cells, and they also secreted minor amount of IL-4 and IL-5. The results indicated that the IFN-γ and TNF-α produced by uNK cells ensured a successful pregnancy progress.