Introduction : One of the factors that influence the incidence of diarrhea in children is environmental factors and maternal knowledge because knowledge determines health behavior. The purpose of this study was to determine the relationship between the level of knowledge of mothers about diarrhea and the occurrence of diarrhea in the Kasun II Posyandu Toddler Village Banyukambang Puskesmas Working Area Wonoasri Madiun. Methods : The research design is correlational analytic with cross sectional approach. The study was conducted on December 11, 2018 at the Kasun II Posyandu Toddler Village Banyukambang Wonoasri Madiun Health Center Working Area. The population is all mothers of children under five in the Kasun II Posyandu Toddler Village Banyukambang Wonoasri Madiun Health Center Area as many as 37 respondents. Sampling uses purposive sampling. A sample of 36 respondents. There are 2 variables, the independent variable is knowledge and the dependent variable is the occurrence of diarrhea. Data collection using a questionnaire. The statistical test uses a contingency coefficient with a significant level of α = 0.05. Results : The results showed that from 36 respondents most of them had less knowledge about diarrhea, as many as 21 respondents (58.33%) and most of the respondents under five had diarrhea as many as 27 respondents (75%). The statistical test of the contingency coefficient obtained p value 0,000 α α = 0.05 so that Ha was accepted. There is a relationship between the level of knowledge of mothers about diarrhea and the occurrence of diarrhea in the Kasun II Posyandu Toddler Village Banyukambang Community Health Center Working Area Wonoasri Madiun. Conclusions : The knowledge that most respondents lack of diarrhea makes the behavior of the respondents wrong. Respondents tended not to care about cleanliness, especially the hygiene of eating and drinking toddlers, which triggered diarrhea in infants.