Water is the most indispensable nature resource in the world for every living being waterborne diseases most commonly are transmitted in contaminated fresh water. Infection commonly results during bathing, washing, drinking, in the preparation of food, or the consumption of food thus infected. Objectives: To assess the Level of Knowledge Regarding Water Borne Diseases and its Prevention among mothers of Under Five Children. To find the association of knowledge among mothers of under five children with selected demographic variables. Methodology: A quantitative approach with Descriptive research design was used. The independent variable in this study Water borne disease and its prevention. The dependent variables in this study are Knowledge of mothers of under five children on waterborne diseases and its prevention. A study was conducted in Erayamangalm village, Thiruvallur District The sample size consists of 60 under five children mothers. Who fulfils the inclusion criteria were selected by Convenient sampling technique sampling technique. Results: Major findings of the study were overall mean score is 26.83 with a mean percentage 89.43% and standard deviation 2.93. A significant relationship was observed between education, food pattern and source of information with the knowledge. Whereas age, occupation, monthly income and type of family do not show any relationship of mothers regarding water borne diseases and its prevention. Result of this study reveals that 8.33% of mothers were having moderate knowledge of water borne diseases and its prevention and 91.67% of mothers were having adequate knowledge on water borne diseases and its prevention. Conclusion: Waterborne diseases being the major health problem in children below five years of age and most of the mothers are unaware about its first aid home management and leads to complications. The study was done to assess the knowledge and practice on prevention of waterborne diseases among mothers of children below five years of age.