The present study was carried out to identify the effect of vaccination time of pregnant ewes withlocally prepared inactivated Rift Valley Fever (RVF ) vaccine on seroprevalence of RVF-IgGantibodies in both dams and their lambs. 60 serum samples were collected from 3 groups of both ewesand their lambs (30 samples from each) and RVF IgG antibodies were detected using ELISA test, Alsoto 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 duringthe last 3 to 6 weeks of gestation. Sera were collected from ewes before vaccination, 2, 8, 12 and16weeks after vaccination and from their lambs at age of one month, 2, 2.5, 3 and 3.5 month of age. Ourresults indicated that RVF IgG antibodies only detected in lambs sera born from ewes vaccinatedduring the last 3 to 6 weeks of gestation. Not all vaccinated ewes had the RVF virus-IgG antibodiesagainst inactivated RVF vaccine. All sera collected from both ewes and their lambs were negative forRT-PCR. Our results concluded that the best vaccination time for ewes was during the last 3 to 6weeks of gestation period to achieve the highest immune response for both ewes and their lambs.