Background: The study was conducted to examine the predic-tive value of serum estradiol and progesterone on the day of hu-man chorionic gonadotropin (hCG) administration and embryo transfer for clinical pregnancy rate in modified natural-cycle frozen embryo transfer (NC-FET). Material and Methods: In a longitudinal prospective study, all eligible women who un-derwentMaterials and Methods: In a longitudinal prospective study, all eligible women who underwent NC-FET cycles with hCG triggering in Royan Institute, Tehran, Iran, from June 1, 2015, to December 31, 2016, were evaluated. Serum estradiol and progesterone levels were measured at menstrual cycle initiation, on day of trigger with hCG, on day of embryo transfer, and in pregnant women every 7 days until the observation of a gestational sac with embryonic heartbeat.Results: In total, 101 modified natural FET cycles were as-sessed, and the clinical pregnancy and live birth rates achieved were 34 (33.6%) and 32 (31.6%), respectively. The demograph-ic and laboratory characteristics of the pregnant and non-preg-nant women following NC-FET were compared together. The mean estradiol level (P=0.041) and size of the dominant follicle (P=0.034) on the day of hCG administration in pregnant women were significantly greater than those of non-pregnant women. In addition, there was a difference between the groups in terms of the mean level of basal serum AMH, but this did not reach the significance level (P=0.075). No significant difference was seen between groups regarding other variables. The changes in estradiol level during early pregnancy showed an increase by an average of 200 pg/mL per week. Multivariable logistic regres-sion analysis showed that only the estradiol level on the hCG day was a significant predictive variable for clinical pregnancy following NC-FET (P=0.04). Conclusion: Serum estradiol level on the day of hCG admin-istration was a significant predictor of clinical pregnancy in modified NC-FET. The study findings demonstrated that estra-diol level on hCG administration day was associated with the developmental competence of the corpus luteum, and therefore an optimal luteal structure-function during the modified natural cycle and the initial phase of pregnancy after FET.