UNICEF in 2013 reported more than 400 children die everyday. The prevalence of diarrhea occurs more in developing countries. Children under the age of five experience an average of 1.6 to 2.3 episodes of diarrhea per year. The aim of the study was to determine the prevalence of diarrhea based on knowledge and environmental sanitation in Tanjung Lago wetland area. This study used cross sectional design with quantitative approach. The sampling technique was simple random sampling with mapping. The sample was 145 housewives with children under five. It used univariate and bivariate analysis. The prevalence of diarrhea in Tanjung Lago was 13.8%. Only a portion of the respondents knew what diarrhea was (53.1%) and the cause of diarrhea (53.8%). The majority of respondents used river water as a source of clean water (91.0%) and refill water as a source of drinking water (98.6%). Most of respondents who have latrines were only less than 10m from daily sources of clean water (91.7%). There is no relationship between knowledge. quality of clean water and availability of latrines with the incidence of diarrhea on children under five. It is important to communicate. inform and educate the public about the environment that can be a medium for diarrhea transmission and the quality of clean water. , 0 (2018) E3S Web of Conferences