Objective: To investigate whether live birth rate (LBR) following frozen-thawed embryo transfer in tamoxifen -stimulated cycles (T-FET) differs from hormone replacement treatment FET (HRT- FET) in women with thin endometrium. Design: Retrospective cohort study. Setting: Tertiary-care academic medical center. Participant(s): A total of 671 patients with thin endometrium who fulfilled the inclusion criteria were involved in the period from January 2016 till February 2019. Methods In the group of T-FET, 20 mg TAM per day was giving from day 5 of the menstrual cycle for 5 days. Day-3 ET was performed four days after ovulation while blastocyst transfer was performed six days after ovulation. In the group of HRT- FET, estradiol val¬erate was taken 6 mg/d from menstrual cycle day 2-3. 12 to 14 days later progesterone 40 - 60 mg/d was given. Embryo transfer was performed 3 or 5 days later for day-3 embryos or blastocysts respectively. Main Outcome Measure(s): LBR per embryo transfer was the primary outcome. The secondary end points included ongoing and clinical pregnancy rate, cancellation rate, endometrial thickness and pregnancy loss rate. Multivariable logistic regression analysis was performed to adjust for potential confounders. Result(s): LBR was significantly higher in T-FET group than HRT-FET group. Moreover, the clinical and ongoing pregnancy rate also higher in the T-FET group than in the HRT-FET group. Conclusion(s): In patients with thin endometrium undergoing FET, tamoxifen use for endometrial preparation was associated with higher LBR compared with HRT cycles. Funding: No external funding was used.