Objective: To present a case of conjoined twins (CTs) in a triplet pregnancy after frozen blastocyst transfer and a literature review. Design: Case report. Setting: Reproductive medicine center of a university teaching hospital. Case Report: A 24-year-old patient underwent in vitro fertilization (IVF) and received two frozen blastocyst transfers. Sequential examination by transvaginal ultrasonography and transvaginal multifetal pregnancy reduction were performed during early pregnancy. Ultrasound images of the fetus in the gestational sac were assessed. Two gestational sacs were found in the uterus at 30 days after the embryo transfer (ET). A germ with primitive heart beat was observed in each sac. At the seventh week of gestation, one heart was still observed in each sac. At the tenth week, a single fetus was observed in one sac, whereas thoracopagus CTs with one heartbeat were seen in the other sac. Selective fetal reduction of the CTs was performed at the 11^th week. Unfortunately, abortion occurred at the 20^th week because of premature rupture of membranes. Conclusions: The authors presented a case of CTs that occurred after the transfer of two frozen blastocysts, and suggested that factors such as ovulation induction, intracytoplasmic sperm injection (ICSI), preimplantation genetic diagnosis (PGD), culture time and culture condition, assisted hatching and blastocyst transfer, and maternal age might be responsible for the CTs. The importance of early transvaginal ultrasound in pregnancies resulting from assisted reproductive technologies (ARTs) is emphasized. Selective embryo reduction is an appropriate choice for the management of these pregnancies.