BACKGROUND: Neospora caninum is an apicomplexan protozoa,which causes abortion in cattle worldwide. OBJECTIVES:The present study was designed to estimate the rate oftransplacental transmission of N.caninum to aborted fetuses inseropositive dams in dairy farms in Mashhad area, Iran.METHODS: Two hundred blood samples of dam with abortedbovine fetuses were collected from dairy farms from 2008 to2010. First , the maternal sera in each aborted case were tested byusing ELISA method, and then the brain samples of abortedfetuses were examined for detecting DNA of N. caninum, usingthe PCR method. RESULTS: In this study, 38 (19%) of dams' serashowed seropositive reaction against N.caninum infection, andalso 23 (10.5%) aborted brain fetuses showed positive reactionby PCR. A significant association was observed betweenN.caninum-serapositie dams and infected aborted fetuses(p<0.0001). In the present study, 20 (86%) infected fetuses wereaborted from seropositive, and 3 (13%) of the fetuses wereaborted from seronegative dams. CONCLUSIONS: Based on theresults of molecular and serology examination, the rate oftransplacental transmission of N. caninum infection in dairycattle were calculated as 52%.