The Cauvery River is the main water source for many places of Tamil Nadu. It is highly polluted in Erode district due to improper management of textile effluents. This study was carried out to analyse the phytoplankton diversity of Cauvery River in TamilNadu, India. Planktons are the basic food source of aquatic ecosystem. Phytoplankton diversity is one of the most important ecological indicators for assessment of water quality. This study was designed to analyse the phytoplankton during period of June 2019 to November 2019 and the results were recorded periodically. The results revealed that the diversity of Phytoplankton is good indicators for the river ecosystem and influenced by quality of river water.