Background: Diarrhea that is not treated fast and accurately can cause dehydration. Dehydration caused by diarrhea becomes the main cause of baby and children mortality in the world. The factors of dehydration risk in toddlers with diarrhea are inaccurate diarrhea treatment in the house, too much vomit, and fever. Research objective:The research aimed at finding out the knowledge level of mothers about the risk factors of dehydration cause in toddlers with diarrhea in the area of Kasihan Health Centre of Bantul, Yogyakarta. Research method: The research was descriptive observational in nature. The sample collecting technique was purposive sampling. The number of the sample was 36 mothers with toddlers that experinced diarrhea. The validity test used was pearson product moment and the reliability test used was KR 20. The data analysis used was univariat analysis. Research result: The knowledge level of mothers about the risk factors of dehydration cause in toddlers with diarrhea categorized as good belonged to 18 respondents (50.0%). The knowledge level of mothers about the definition and risk factor of diarrhea treatment in the house which was not accurate as the risk factor of dehydration cause in toddlers with diarrhea categorized as good belonged to 20 respondents (55.6%). The knowledge level of mothers about the definition and risk factor of too much vomit as risk factor of dehydration cause in toddlers with diarrhea categorized as good belonged to 18 respondents (50.0%). The knowledge level of mothers about the definition and risk factor of fever as risk factor of dehydration cause in toddlers with diarrhea categorized as good belonged to 18 respondents (50.0%). Conclusion: The knowledge level of mothers about the risk factor of dehydration cause in toddlers with diarrhea in the area of Kasihan Health Centre II of Bantul Yogyakarta was good.