Abstract: Objective To study the expression of uncoupling protein 2 (UCP2) mRNA and protein in the endometrium and its changes during periimplantation period in relation to infertility. Methods In situ hybridation and Western blotting were used to detect 21 endometrium specimens from normal cycling women during different menstrual phases, 22 secretory endometrium specimens from infertile women and 7 decidual and chorionic villus specimens. Result UCP2 mRNA and protein were detected in the endometrium and decidua besides, the trophoblast, and UCP2 level in the endometrium increased during pregnancy. Compared with normal women, women with hydrosalpinges had significantly stronger endometrial UCP2 expression in the mid-luteal stage. Conclusions UCP2 down-regulates reactive oxygen species and plays a role in the regulation of inflammatory events to prepare for embryo implantation. Strong endometrial UCP2 expression might be an important reason that hydrosalpinges affect embryo implantation.