Nutrient concentrations and phytoplankton biomass were monitored in the upper Euphrates River. The purpose of this study was to obtain detailed information about the composition variations in the phytoplankton community, and to determine the driving force for the succession of the dominant genera. Cyanobacteria were predominant with 76.88 % of the total abundance in the Karakaya Dam Lake. Chlorophyta (7.81 % of the total abundance) was the most abundant group, followed by Bacillariophyta (7.72 %), Dinophyta (7.56 %), and Euglenophyta (0.03 %). The phytoplankton seasonal sequence was mainly correlated with environmental factors. Unimodal species distribution was shown in UPGMA cluster analysis, classified into the two cluster groups. RDA was used to examine relationships between measured environmental variables and phytoplankton populations of the study area. Eigenvalues of axes explained 100 % of the cumulative variance in species data and 99.6 % of the relationship between species and environmental data. According to water quality, DIN: SRP ratio was >16:1. Water temperature and conductivity were main driving forces for the seasonal succession of the abundance. Based on RDA-Pearson correlation analysis, the results showed that the phytoplankton abundance and biomass were positively correlated with the changes of environmental factors (p<0.01, p<0.05). Except for DO, Secchi, SRP, nitrogen, and pH, there were no correlations with the phytoplankton abundance and biomass. It is important to control the exogenous nutrient in order to maintain ecosystem healthiness of Karakaya Dam Lake.