Japanese encephalitis is considered an essential disease-causing swine reproductive failure in Asian countries; however, significant knowledge gaps remain about this problem in actual cases. This study examined 55 cases of reproductive failure in sows, including one abortion and 54 full-term farrowing litters, to find the Japanese encephalitis virus (JEV) and specific antibodies against JEV. Haemagglutination test was used to detect specific antibodies against JEV from 63 samples of fetal thoracic fluids and newborn un-suckling piglet sera (54 thoracic fluid samples of dead fetuses, two thoracic fluid samples of abortion fetuses, seven sera samples of weakly newborn un-suckling piglets), viral isolation and RT- PCR technique was attempted from 60 brains of fetuses and newborn un-suckling piglets, histopathology sections of fetal brains which were positive with JEV confirmed by HI test and RT-PCR were also made. The results showed that 17.46% (11/63) of thoracic fluids and sera were positive for JEV. No JEV isolation was found from 60 brain samples of dead fetuses and piglets, but RNA of JEV were detected from 5 of them (8.33%). Our results suggest that JEV should be considered the important cause of swine reproductive failure in Viet Nam.