The purpose of this study was to analyze fish fauna and species compositions along with trophic guilds, tolerance indicators, and fish community conditions before weir construction (BWC) and after weir construction (AWC) in the Yeongsan River watershed. Total 45 and 44 fish species were sampled in BWC and AWC, respectively without any distinct differences through weir constructions. Fish fauna analysis revealed that the dominant species was the same, Zacco platypus with 24.3% and 20.8% in BWC and AWC, respectively. However, the subdominant species were Carassius auratus with 8.9% in BWC but Opsarichthys uncirostris amurensis with 20.3%, almost identical that of the dominant species AWC. This phenomenon showed the distinct modification of species composition in the watershed. We sampled the $1^{st}$ class endangered species, Liobagrus obesus in tributary stream as previously reported. Also Culter brevicauda was sampled in the mainstream of Yeongsan River watershed and this was the first sampled record in this watershed. One of the most important features were an increase of exotic species, such as Micropterus salmoides and Lepomis macrochirus, with 3.2% BWC vs. 10.2% AWC as well as the increase in tolerant species with 49.2% BWC vs. 73.7% AWC, indicating ecological degradation through weir construction. Overall, our results indicated that fish fauna and composition analyses showed distinct ecological degradations related to increases of exotic and tolerant species AWC. Further long-term studies of fish monitoring should be conducted in the future to configure existent status of river conditions and to provide key information in order to conserve the healthy ecosystem.