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 childhood illness would help to guide strategic planning and prioritize interventions.Method: Community based cross-sectional study was conducted. A two-stage cluster sampling technique was used to select households and mothers/caregivers. A total of 2,158 mothers with at least one under-five child were included. Mothers were interviewed at their homes using structured questionnaire. Adjusted Odds Ratio with a 95% Confidence Interval and P-value <0.05 in the multivariable model were reported to show the significance and direction of association.Result: The overall prevalence of childhood illnesses was 16.5% (95% CI: 15.0, 18.2). Of all, 6.5% (95% CI: 5.5, 6.72), 7.5% (95% CI: 6.5, 8.76), and 12.0% (95% CI: 10.66, 13.40) had diarrhea, ARI, and fever, respectively. The treatment-seeking behavior of mothers/care-givers for any of the childhood illness was 22.7% (18.72, 27.42). The odds of childhood illness was higher among mothers’ age 15-19 years (AOR=2.87-; 95% CI: 1.33, 6.13), employed mothers’ (AOR=2.28; 95%CI: 1.84, 4.39), living far away from nearest health center (AOR=1.02; 95%CI: 1.01, 1.03), two under-five children (AOR=1.33;95%CI: 1.029,1.74) and three or more children in the house (AOR=2.70; 95%CI: 1.12, 6.53). About 22.8% (95% CI: 18.7, 27.4) of mothers/caregivers sought health care. Mothers’ occupation (AOR= 4.08 (95% CI: 1.35, 12.39) was the only independent predictor of treatment-seeking behavior.Conclusion: In this study, common childhood illnesses remain high while the treatment-seeking behavior of the mothers’/caregivers’ was low. Mothers’ age, occupation, distance from the nearest health center, and two or more children in the house were independent factors of childhood illness. Being a skilled/semi-skilled mothers was significantly associated with health-seeking behavior of mothers/caregivers. Thus, involving women in skilled/semi-skilled occupations would help in decreasing childhood illness and enhance health-seeking behaviors for their under-five children. Besides, providing access to health care facilities in their nearby area would be essential in reducing childhood illness.