In the present study, it was aimed to investigate the effect of vitamin D injection on lambing performance and some fertility parameters on the day the synchronization protocol was started in Romanov sheep in breeding season. The study was carried out in 40 Romanov sheep aged 2-4 years. The sheep in the first group (n=23) were injected with vitamin D (1 ml, Provet-D3®, Provet, Turkey) on the day the synchronization protocol was started, while the sheep in the second group (n=17, control group) were injected intramuscularly with 1 mL of saline. In order to synchronize the estrus, intravaginal sponge (20 mg flugestone acetate, Chronogest®, France) was applied to all sheep for 14 days and 300 IU pregnant mare serum gonadotropin (eCG; Chrono-Gest/PMSG®, Germany) was injected intramuscularly immediately after removal. Estrus (100%), pregnancy (100%) and lambing rates (100%) were similar in both groups. It was determined that the rate of multiple lambing was higher in the vitamin D administered group. Fecundity and productivity rates were also higher (91.3%) in the vitamin D administered group. As a result, it is thought that the application of vitamin D may contribute to the increase in the yield of offspring to be obtained from the synchronization protocol. It has been concluded that more comprehensive studies are needed to more clearly reveal the effects of vitamin D injections on fertility parameters. [ABSTRACT FROM AUTHOR]