Experimental duel infection with S. mansoni and E. granulosus was induced in mice to determine their effect on serum nitric oxide (NO) level and accordingly on the sequences of histopathological lesions affecting the liver. The results showed that serum NO level was significantly increased (p<0.05) in mice infected with both parasites (GI) in comparison to either S. mansoni (GIV) or E. granulosus (GV). The NO elevation on hepatic pathological lesions of both diseases showed a marked reduction of granuloma size with absence of concentric fibrosis in GI as early as 4 weeks of concomitant infection as compared to GIV. In spite of the significant increase of NO level when E. granulosus infection induced in late stages of schistosomisais (GsII & III), yet granuloma size was not suppressed. Also, there was absence or death of hydatid cyst in mice (GI) compared to E. granulosus (GV). So, the duel infection with the two parasites affected serum NO level and hepatic histopathology, by ameliorative or deteriorative effects, according to duration of infection with either.