Diversity, relative abundance and some biological aspects of fishes in Geba and Sor Rivers were studied using gill nets (6, 8, 10 and 12cm stretched mesh sizes) and hooks. Samples of fish were collected in wet (October to November 2012) and dry (February to March 2013) seasons. A total of 348 fish specimens were collected from both rivers at both seasons. Nine fish species were identified which were included in six genera and four families. The represented families include Mormyridae, Cyprinidae, Bagridae and Cichlidae. The family Cyprinidae was the most dominant with respect to number of species consisting of six species (66.7%). The diversity of fish species in Geba River (H’=1.50) was higher than that of Sor River (H’ =1.10). The fish diversity of Geba and Sor Rivers is less compared to most studied Ethiopian rivers. Labeobarbus intermedius (60.72%IRI), Labeobarbus nedgia (16.83%IRI) and Labeo cylindricus (14.66%IRI) were the most abundant fish species. The abundance of fish specimens in dry season was higher than in wet season. The length-weight relationships for these species were found to be curvilinear. The mean FCF for Labeobarbus intermedius, Labeobarbus nedgia and Labeo cylindricus were 1.21, 1.02 and 1.21 in Geba and Sor Rivers, respectively. There was significant variation (ANOVA, P<0.05) in FCF of Labeo cylindricus in the two seasons. However, variations were insignificant (ANOVA, P > 0.05) for the Labeobarbus intermedius and Labeobarbus nedgia in wet and dry seasons. In both rivers females were more numerous than males and statistically significant (Chi-square, P < 0.05). Further investigation on the fish diversity especially on the tributaries of these rivers and socio-economic aspects of the two rivers is recommended.