Background:Although there is global decline in the death rates of children younger than 5 years old, the risk of a child dying before becoming 5 years of age remains highest in the WHO African Region (90 per 1000 live births). The highest rates of child mortality are seen in sub-Saharan Africa, where 1 in 8 children dies before age 5, more than 17 times the average for developed regions. The problem of child death in Ethiopia is worse, with an Ethiopian child being 30 times more likely to die by his/her fifth birthday than a child in Western Europe. Therefore, the aim of this study was to assess the prevalence and factors associated with diarrhoea among children younger than five years old.Objective: The objective of this study was to measure the prevalence of diarrhoea and its associated factors among children under-five years of age in Serbo town in Jimma zone, Ethiopia. Methods:A community-based, cross-sectional study was conducted on children younger than five years old from June to July 2017. A total of 250 under-five children were included in this study, and subjects were recruited by the multistage cluster sampling technique. Data were collected by a pre-tested questionnaire. The data were coded, entered, cleaned and analyzed the SPSS software package version 24. Result:The prevalence of childhood diarrhoea was 14.9% (n=26) high rate in under 2 years and no sex variation. The proportion of acute watery diarrhoea was 92.3% (n=24) of those children 76.9% (n=20) were treated in health institution. The age care takers was assessed and those 20-39 years of age had the highest rate 65.4% (n=17) of the two weeks diarrhoea prevalence. Mothers of children who had completed 1-6 grade had the highest rate 11 (42.3%) of two weeks diarrhoea prevalence. Conclusion:In this study, prevalence of diarrhoea among under-five children was quite high. The highest rate of the prevalence was significantly concentrated among children aged 20-29 (40.1%) months. The age of child, educational level of mothers, and the economic status of households were the most important variables that affected the occurrence of diarrhoea in children.