Diarrhea is a disease based on the environment. serviço saùde Muncipio Covalima is one of the regions with the second-highest IR in Timor-Leste 2017, with a prevalence of 16.7 percent of all diarrhea events and a major cause of under-five deaths of 25.2 percent. The purpose of this study was to determine the relationship between environmental sanitation and maternal behavior with the incidence of diarrhea in infants in the Serviço Saude municipal area of Covalima. This research is quantitative analytic research with a Cross-Sectional approach. The sample in this study amounted to 98. The technique in sampling; Multi-stage Random Sampling. Research instruments in the form of questionnaires and observation sheets. The results showed that; "There is a meaningful relationship between drinking water sources with Prevalence Ratio (RP) 1,183 and 95% CI: lower 1,038 and upper 13,421, latrine ownership with Ratio Prevalence (RP) 1,389 and 95% CI: lower 1,705 and upper 22,302, behavior/habits cook or boil drinking water and process food with a Prevalence Ratio (RP) 1.119, and 95% CI: lower 1.634 and upper 17.706, and behavior / habits of providing complementary feeding and caring for children with Prevalence Ratio (RP) 3,238 and 95% CI: lower 4,900 and upper 183,690. The results of this study also showed that: There was no relationship between the availability of trash cans and household sewage, behavior/habits of defecation, behavior/habits of washing hands with soap with the incidence of diarrhea in children under five. Behavior/habits of providing complementary feeding and caring for children are the most significant factors associated with the incidence of diarrhea in infants. It is recommended to the community, especially mothers who have toddlers to maintain environmental conditions and improve clean and healthy living behavior, especially in taking precautions to prevent diarrhea such as washing hands with soap after bowel movements, before eating, before preparing and feeding toddlers.