To determine effect of the administration of exogenous melatonin and photoperiod on initiating the lambing season and improving reproductive efficiency of sheep, a total of thirty-four multiparous, non-pregnant Ossimi ewes aged 3-5 years with average body weight 55.94±1.80 kg were used. After weaning and the starting of the breeding season, the ewes were divided into four groups; the 1st group (7 ewes) was exposed to natural daylight (NL) and severed as a control group, the 2nd group (9 ewes) was exposed to 16-hour artificial lighting (AL), the 3rd group (10 ewes) was exposed to natural daylight+18-mg/h melatonin (NL+MEL), while the 4th group (8 ewes) was exposed to 16-hour artificial lighting+18-mg/h melatonin (AL+MEL). The treatment of the ewes continued throughout the mating season and for three consecutive estrus cycles (60 days). Ewes were weighed before mating and after parturition. Reproductive traits were recorded after parturition. Results revealed that exogenous melatonin (induced by either natural daylight or artificial photoperiod) exhibited significantly (P≤0.05) higher body weights (6.20±1.711 and 4.14±1.91kg for NL+MAL and AL+MEL groups more than the control group). Both of the melatonin treatment and the use of artificial light for 16 h insignificantly influenced reproductive parameters (percentages of fertility, conception rate, scanning, lambing rate, and fecundity rate of Ossimi ewes. Fertility and conception rates tended to be higher in (NL+MEL) and (AL+MEL) groups, while SP percentage, lambing and fecundity rates were higher in treated groups than control. The percentage of the number of services per conception (NSC) and abortion rate were insignificantly reduced in treated ewes as compared with the control. Both weaning rate and the survival rate was elevated (P≥0.05) in treated ewes than untreated ones. Lambing interval and days open were significantly lower (P≤0.05) in AL+MEL group than AL and NL+MEL groups. The lambs' weight at birth, weaning, net gain, and average daily gain tended to be higher in (NL+MET) and (AL+MEL) groups than other groups with insignificant differences. It can be concluded that, stimulation of melatonin (induced by natural daylight and artificial photoperiod manipulation) might be a useful tool in improving reproductive performance and lamb production in Ossimi sheep.