A total of twenty-four healthy ewes aged between 2-3 years old and weighed about 35.09 kg of body weight were divided randomly into four groups (6 animals for each group). The first group included the non-treated control group, while the other three groups were subjected to various oral doses of melatonin 9 mg, 12 mg and 15 mg/head, respectively. All treatments were applied at 6 o'clock in the morning, to investigate the effect of treatment with melatonin during breeding season on reproductive performance and levels of some biochemical parameters in Arabi ewes from 1 May to 1 October 2017. Blood samples 7ml were collected after one hour of treatment and then monthly during the pregnancy months to determine the concentrations of biochemical parameters. After Postpartum, lambed ewes/lambs number, birth weight, placenta empty weight and cotyledon number were recorded. The results showed significant (P<0.05) improvement in reproductive performance (fertility percentage, twinning rate and the number of offspring) when ewes administrated melatonin at two doses (12 and 15 mg/ head). Also, melatonin administration caused significant (P<0.05) decrease in the concentrations of thyroxin, cortisol, and cholesterol. Conversely, there was a significant increase (P<0.05) in glucose concentration after 1 hour of treatment. The concentrations of thyroxin, cortisol, cholesterol, and vitamin C increased significantly (P<0.01) in early-gestation compared with mid and late-gestation, while the concentrations progesterone and glucose increased significantly (P<0.05) in midgestation compared with early and late-gestation. In conclusion, melatonin administration at doses 12 and 15 mg/ head improves ewe reproductive performance (fertility percentage, twinning rate and the number of born lambs). Additionally, the gestation stage has a negative effect on the concentrations of thyroxin, cholesterol and vitamin C.