Studies on diatom of river Cauvery water, Mettur and Bhavani Salem District, Tamil Nadu was made to assess the pollution of water from May 2011. Our work was focused on Physico-chemical and environmental studies of Cauvery River in parts of Tamil Nadu. Macrophytes and water samples were collected from unpolluted and polluted sites along Cauvery River course, some ions from water also analyzed. Analysis of diatom communities has been performed in order to quantity the level of pollution in Cauvery River. The present study revealed that the water of Cauvery in Mettur and Bhavani are highly polluted by direct contamination of sewage and other industrial effluents. The source of pollution is most probably the dying factory and sewage from Kumarapalaiyam. The structure of diatom communities is strongly influenced by the different pollution source from this area.