Abstract: Phytoplanktons which are present in a variety of aquatic habitats were studied with respect to their species diversity and distribution, studies on phytoplankton and water quality of Hadinaru lake , Nanjanagudu, Mysore, Karnataka were undertaken for 5 months of 2022 (April 2022 to August 2022). . The water samples were analyzed to physical and chemical parameters by following the standard methods of APHA (2005) and Trivedi and Goel (1984). The quantitative analysis of phytoplankton was done by Lackey’s drop method modified by saxena (1987). Diversity indices have been discussed by using PASTA Software Program. Among the total 52 species recorded during the study, 25 were from Bacillariophyceae, 8 from Chlorophyceae, 8 from Cyanophyceae. 4 from Desmidiaceae 3 from Euglenophyceae and 2 from Charophyta. Bacillariophyceae was the most dominant group followed by Chlorophyceae, Cyanophyceae, Desmidiaceae. Euglenophyceae and Charophyta. The result of physico- chemical and biological parameters along with statistical method and biodiversity indices indicated that the Hadinaru lake is threatened ecologically due to various anthropogenic activities which lead organic pollution and eutrophication status of the lake. Statistical program Bray – Curtis similarity index explained, the cluster of EC, Total hardness and Total alkalinity is highly correlated with the clusters of phytoplanktons. Shannon and Weiner index showed that the lake was heavily polluted in the month of April due to increased temperature.