Background: In most developing countries, childhood illness and deaths are among the most challenging health issues. Most of these deaths were due to preventable causes including acute respiratory infections (ARI), diarrhea, and febrile illnesses. Thus, identification of determinants of these childhood disease illness would help to guide strategic planning, and prioritize interventions. Method: Community based cross-sectional study was conducted. Two stage cluster sampling technique was used to select kebeles and then households. A total of 2,158 mothers/care givers with at least one under-five child were included. Mothers/care-givers were interviewed at their homes using a pretested and structured questionnaire. Adjusted Odds Ratio (AOR) with a 95% Confidence Interval (CI) for variables with P-value <0.05 in the multivariable model were reported to show significance and direction of association<0.05 in the multivariable model were reported to show significance and direction of association. Result: The overall prevalence of childhood illnesses was 16.5% (95% CI: 15.0, 18.2). The treatment seeking behavior of the mothers/caregivers was 22.8% (95% CI: 18.7, 27.4). Of the illnesses, 6.5% (95% CI: 5.5, 6.72) were diarrhea, 7.5% (95% CI: 6.5, 8.76) were ARI, and 12.0% (95% CI: 10.66, 13.40) were fever. Mothers’ age 40 years and above (AOR=0.35; 95%CI: 0.16, 0.76), mothers’ occupation (AOR=2.28; 95%CI: 1.84, 4.39), distance from nearest health center (AOR=1.02; 95%CI: 1.01, 1.03), and presence of three or more under-five children in the house (AOR=2.59; 95%CI: 1.03, 5.76) were predictors of childhood illness. Conclusion: In this study, the common childhood illness remains high while treatment seeking behavior of the mothers’/care givers’ was so low. Mothers’ age and occupation, distance from nearest health center, and presence of three or more under-five children in the house were independent determinants of childhood illness. Thus, addressing significant determinants and enhancing mothers’/care givers health seeking behavior would help in reducing morbidity and mortality of under-five children with common childhood illness. Key word: Childhood illness, determinants, rural, health seeking behavior, Ethiopia