Aras River is one of the largest rivers of Caspian Sea Basin in Northwest of Iran which plays an important role in regional water supply for various uses. This study executed in area from Siahrood to Aras Dam Lake in the summer of 2017. Sampling, identification, enumeration and recording demographic changes in different terms of growth season was performed for each of 7 stations for three months. Identified genera (32 genera) belong to Bascillariophyta, Chlorophyta, Cyanophyta and Dinophyta. Diatoms with 18 genera, chlorophyta with 8 genera, Cyanophyta with 5 genera and then Dinophyta with one genus were most dominant phylum in Aras River, respectively. The highest algae density at the beginning and end of the growing season was due to the Navicula genus of Bascillariophyta and the Oedogonium genus of the Chlorophyta phylum.