In assisted reproductive technology (ART), hormone replacement treatment embryo transfer (HRT-ET) is common. However, the appropriate serum progesterone concentration is not well known in the case of transvaginal progesterone administration. The purpose of the current study is to investigate the relationship between serum progesterone concentration and pregnancy rate in HRT-ET using progesterone vaginal tablets. This is a retrospective cohort study, the authors investigated all patients who underwent HRT-ET at this hospital for the first time from September 1, 2010 to September 30, 2015. A total of 227 cycles were performed in all 123 patients who underwent HRT-ET. In the total 123 cycles of first time HRT-ET, 32 cycles (26.0%) developed a pregnancy. The results of the multivariate analysis indicated that the patient’s age, embryo grade, serum estradiol concentration, and progesterone concentration had significant differences in relation to the pregnancy rate (p < 0.05). The latter rate tends to be lower when the serum progesterone concentration is less than 5 or more than 15 ng/ml. Therefore, the authors suggest that the appropriate serum progesterone concentration ranges from 5 to 15 ng/ml in HRT-ET using progesterone vaginal tablets.