Abstract: The objectives of this study were to determine the effect of administration of exogenous hCG 5 days after Artificial Insemination (AI) on serum progesterone concentration and conception rate in lactating dairy cows. In this experiment 5 days after AI the cows were alternately assigned randomly to treatment group (n = 70) received an injection of 3000 IU hCG (Chorulon, each vial contained 1500 IU, Intervet, Holland) and control group (n = 88), received no treatment. On day 5 and 12 blood samples from 20 of cows (10 in each group) were obtained to measure serum progesterone concentrations. The study population was stratified by parity, milk yield production, number of AI and Days in milk. Cows that were detected in estrous after days 18-24 were reinseminated and recorded as no pregnant (open) to the prior AI. The uteri of cows not observed in estrous were palpated per rectum 45-50 days after insemination to determine pregnancy status. Pregnancy proportion in treatment and control groups were 27.5 and 35.2%, respectively. The results demonstrated that there was no significant differences between two groups (p<0.05). Mean serum progesterone concentrations did not differ between treatment and control groups on day 5 (2.94±0.65 versus 2.98±0.36 ng mL-1). On day 12 serum progesterone concentrations were greater (p<0.05) in the treatment group compared with control group (16.07±3.64 versus 8.71±0.86 ng mL-1). Regardless of treatment, parity, milk yield production, number of AI and days in milk had no effect on the pregnancy rate. In conclusion, 3000 IU of hCG 5 days after insemination did not improve pregnancy rate in dairy cows, despite the fact that serum progesterone concentration were higher in treatment group.