Background: Globally, 4 billion cases of diarrhea occur every year and 88% of these can be attributed to unsafe water, inadequate sanitation and poor hygiene. One out of ten babies born in developing countries do not reach their fifth birthday as they die from diarrhoeal diseases. Diarrhoeal diseases remain the number one cause of child mortality in India even though it is a preventable illness. The study aims to compare the prevalence of diarrhoeal diseases and enumerate its risk factors among children aged below 5 years in different districts of Tamil Nadu from NFHS 4 and 5. Methods: This study is a secondary data analysis based on the NFHS 4 & 5 data of Tamil Nadu, which has data available in round 4 for 26,033 households and for 27,929 households in round 5. The analysis pertains to children aged below 5 years who were 7,658 in round 4 and 6,283 in round 5. Results: Overall there was a 53.8% decrease in prevalence in Tamil Nadu. Prevalence in Salem had reduced by 92.1%. Erode, Kanniyakumari and Krishnagiri achieved 84.5%, 86.6%, 89.2% reductions respectively. Nagapattinam and Virudhunagar experienced a higher number of prevalence with 7.5% and 12% increase of cases respectively. Overall Tamil Nadu 71.6% improvement in sanitation from NFHS-4. Tiruvannamalai achieved the highest improvement (96.5%), while Erode did not achieve much improvement (7.8%). Overall Tamil Nadu had 0.92% improvement in drinking water facility from NFHS-4. Nagapattinam and Perambalur have the highest improvement (6.8 and 6.5% respectively) and have achieved 100%. Ariyalur has had a 6.1% decline in quality of drinking water facility. Conclusion: Between NFHS 4 and 5, the prevalence of diarrhoeal cases among children under 5 has dropped by 53.8% and child mortality has reduced by 16.8% in Tamil Nadu. We can also see that there is an overall improvement in of 27.4% in sanitation and 0.92% in drinking water quality.