The research was piloted in Atshan river at the city of Samawah for a period of one year, starting from October 2020, monthly samples were taken and included some ecological factors dissolved oxygen varied between 9.6 mg. l-1 in winter and 4.9 mg. l-1 in summer, while the pH was recorded in moderate ranges tending to light basal during the different seasons, as it was within the range 7.7 in winter and 1.7 in spring. As for salinity, it varied between 2.5 parts per thousand in the summer and its lowest level in the winter was 2 parts per thousand. As for water temperatures, it varied between 32.5 degrees Celsius in the summer, and its lowest value was recorded in the winter 14.1 Celsius. Fish number caught was 1055 fish belonging to 12 species Occurrence shows that resident group includes four species and the seasonal group includes three species, whereas the rare group includes five species. Dominance index (D3) value reached 77% for the three most abundant species, O. aureus, C. auratus and C. luteus. Richness index (D) vary between 1.34 in autumn and 1.51 in summer, while Diversity index (H) records highest value in summer 1.68 and lowest in Spring 1.34. Evenness index (J) varied in summer 0.86 and 0.64 in winter. The dominance of exotic species in the Atshan River, represented by blue tilapia and Prussian carp, and a decrease in the number and abundance of local species, and the absence of some of them in the fishing sample, such as carp and brown, which threatens some species with extinction, as well as a significant decrease in the evidence of the biodiversity of Atshan river fish as recorded in the river An increase in salinity compared to the waters of the Euphrates River in Samawa. This research deals for the first time with biodiversity in Atshan river.