Fish diversity is a good indicator of any aquatic ecosystem. An effort has been made to study the fish diversity of Budhi Gandak river, one of the main tributaries of river Ganga. The work has been conducted between January, 2016 and March, 2017. During the study, 44 fish species belonging to 26 Genera, 13 Families and 7 Orders have been identified. Cypriniformes was found the dominated Order with 17 species (39%) followed by Siluriiformes 11 species (25%), Perciiformes 7 species (16%) and minimum of 2 species (4%) of Clupeiformes and Osteoglossiformes. The status of the species included 5 as vulnerable one. The present study showed that Begusarai possesses rich fish diversity but proper conservation measures are required to maintain sustainability and richness of the species diversity of the district.