OBJECTIVE: To confirm endometrial receptivity by evaluating the relationship between endometrial thickness on the day of human chorionic gonadotropin (hCG) administration and clinical outcome in natural cycle frozen embryo transfer (FET) without artificial endometrial preparation. DESIGN: Retrospective study. MATERIALS AND METHODS: 181 cycles from 150 patients who underwent single FET at our clinic between January 2005 and January 2008 were studied. Rapid freezing protocol (Vitrification) was performed on all embryos and those were rapidly thawed, and a single good quality blastocyst-stage embryo with grade greater than 3 without assisted hatching technique was transferred in a natural cycle. Endometrial thickness was measured by transvaginal ultrasound on the day of hCG administration. The cycles were divided into 5 groups of A (<8mm, n=25), B (8-9mm, n=64), C (10-11mm, n=55), D (12-13mm, n=28) and E (≥14mm, n=9) according to endometrial thickness, and clinical pregnancy and miscarriage rates were compared between the groups. RESULTS: There were no significant differences between the groups in age, basal serum FSH level, endometrial echo pattern (triple-line or not triple-line) and rates of luteal phase supports. Endometrial thickness on the day of hCG administration ranged from 5.8mm to 17.9mm. The clinical pregnancy rates among the 5groups were A-24.0% (6/25), B-35.9% (23/64), C-41.8% (23/55), D-57.1% (16/28) and E-33.3% (3/9), respectively. The miscarriage rates were A-0% (0/6), B-8.7% (2/23), C-13.0% (3/23), D-18.8% (3/16) and E-0.0% (0/3), respectively. The clinical pregnancy rates increased as the endometrium thickened (<14mm) and significantly higher rates were achieved in groups C and D compared with those in group A (p<0.05). There were no statistically significant differences in the miscarriage rates between the groups. CONCLUSIONS: Endometrial thickness is an important indicator to evaluate endometrial receptivity for an embryo. The clinical pregnancy rates increase gradually in proportion to endometrial thickness in natural cycle FET. The miscarriage rates do not appear to be related to endometrial thickness. However, this study is not able to establish a threshold of endometrial thickness indicative of unlikely pregnancies in natural cycle.