The present study was carried out to identify the effect of vaccination time of pregnant ewes with locally prepared inactivated Rift Valley Fever (RVF ) vaccine on seroprevalence of RVF-IgG antibodies in both dams and their lambs. A total of 60 serum samples were collected from 3 groups of both ewes and their lambs (30 samples from each) and RVF IgG antibodies were detected using ELISA test, Also to detect RVF virus by RT-PCR. The 1st group include dams vaccinated one week before pregnancy, 2nd group was vaccinated during the 1st 2 month of gestation, while 3rd group was vaccinated during the last 3 to 6 weeks of gestation. Sera were collected from ewes before vaccination, 2, 8, 12 and16 weeks after vaccination and from their lambs at age of one month, 2, 2.5, 3 and 3.5 month of age. Our results indicated that RVF IgG antibodies only detected in lambs sera born from ewes vaccinated during the last 3 to 6 weeks of gestation. Not all vaccinated ewes had the RVF virus-IgG antibodies against inactivated RVF vaccine. All sera collected from both ewes and their lambs were negative for RT-PCR. Our results concluded that the best vaccination time for ewes was during the last 3 to 6 weeks of gestation period to achieve the highest immune response for both ewes and their lambs.