This randomized double blinded clinical trial is conducted to evaluate the effect of intrauterine injection of human chorionic gonadotropin (hCG) before frozen embryo transfer (ET). The study protocol is approved by the Ethics Committee (Institutional Review Board) of Royan institute. The study is performed according to the Declaration of Helsinki for medical research. All participants provide informed consent after explaining the purpose of the study. All the patients with primary infertility who have only one fresh implantation failure and undergoing frozen embryo transfer cycles are enrolled. In all patients, endometrial preparation is performed by hormonal replacement method. In this way, endometrial preparation is started from the second or third day of menstrual cycle with daily administration of 6 mg oral estradiol valerate (Estraval®, Aburaihan CO, Tehran, Iran) for 8 days. After 8 days of estradiol administration, if favourable thickness of endometrium (≥ 8 mm) is confirmed by ultrasound, estradiol valerate is continued with the same dose and 50 mg progesterone (Progestin®, Aburaihan Pharmaceutical. Co., Tehran, Iran) intramuscularly is administered for 3 days and then embryos are transferred. Otherwise the dosage of estradiol is increased to 8 mg/day until the favourable thickness of endometrium be achieved. In the Embryo transfer day, the eligible patients are randomized using sealed opaque envelopes into three groups. In all study groups, the patient is put in the lithotomy position, and the cervix is visualized using Cusco's speculum. The cervical mucous is wiped out using a sterile piece of gauze, and then the mucous is partially removed by gentle suction with a 1-mL syringe. Embryo transfer is performed using a soft catheter (Labotec, Gottingen Germany). After the catheter is passed the internal cervical os; in group A (experimental group) 40 μL of tissue culture medium (G.2plus ref. 10132, Vitrolife) containing either 500 IU of hCG (Choriomon®, IBSA SA, Switzerland) is injected intrauterine. In group B (the placebo group) only 40 μL of tissue culture medium (G.2plus ref. 10132, Vitrolife) is injected intrauterine. In both groups, approximately 7 minutes after injection, the embryos are loaded into another ET catheter and are transferred into the uterine cavity. In group C (the control group), the ET is done without the intrauterine hCG injection. The ET procedure is performed just like in the other two groups.