Objective: Low implantation rate in ART implies the necessity for accelerating endometrial receptivity. Progesterone has commonly been used for this purpose. In the present study the ultrastructural characteristics of endometrial luminal epithelium has been used to evaluate and compare the effect of progesterone and Viagra on endometrial receptivity in pre-implantation stage. Materials and Methods: Forty adult female Balb-c mice were divided into one control and three experimental groups. In 3 experimental groups the mice received 7.5 IU hMG and later hCG. Then every two female mice with one male mouse housed in one cage for mating. In one of the experimental group 1mg progesterone/mouse and in the other 3mg/kg Viagra was administered in 24 48 72 hour intervals after hMG injection. Ninety six hours after hMG injection the mice in all groups were sacrificed and from those that their uterine tube contained blastocyst the uterine specimens were collected and prepared for electron microscopy (LEO 906). Results: Microscopy showed that in the control group luminal epithelial cells were high columnar cells containing euchromatin nuclei numerous microvilli several mitochondria at apical cytoplasm and many subnuclear granules. In experimental groups the cells by having short microvilli several supranuclear granules and development of pinopodes were different than control group. However in case of pinopodes; the group that received gonadotropin + progesterone was similar to control group but the group that received gonadotropin and Viagra were similar to the group that had received only gonadotropin. Conclusion: It is concluded that Viagra may promote some other aspects of endometrium which are considered as increased endometrial receptivity.