BackgroundMalaria can be prevented using cost-effective interventions. It can be prevented at large via the use of insecticide-treated mosquito nets (ITNs). The use of ITNs decreases malaria mortality rates by 55% in under-5 years old children in Africa, Ethiopia, realizing the effectiveness, scaling up distribution and utilization of ITNs to cover 100% of children less than 5 years of age. However, little is known about ITNs utilization and factors associated with the utilization in under-5 years old children in the study area yet. The purpose of this study was to assess the level and associated factors of ITNs utilization in under-5 years old children among households with under-5 years old children of Mirab Abaya District, Gamo Gofa Zone, Ethiopia.MethodsA community-based cross-sectional study was conducted during August–September, 2016. Six study Kebeles were identified by simple random sampling technique and 398 households with at least one under-5 years old children were selected by random sampling technique using computer generated random numbers from health post family folders. Structured, interviewer questionnaire was administered to mothers or care givers of the children. Data were entered to Epi Info Version 3.5 and analyzed in SPSS version 21 statistical software. Bivariate and multivariate logistic regression analysis was done. P-value <0.05 and odds ratio with 95% confidence interval were used for the determination of associations between dependent and predictor variables. Results were presented in narrations, tables, and graph.ResultAmong 398 under-5 years old children assessed, the majority, 362 (91.0%) of them had access to ITN, but only 137 (37.2%) of the child had ITNs utilization during the previous night prior to the survey. Households with age of mothers or caretakers 31–44 years, AOR = 0.03, 95% CI (0.01–0.07) and ≥45 years of age; AOR = 0.05, 95% CI (0.01–0.58); households with family size ≤5 members, AOR = 11.23, 95% CI (4.31–29.24); and households with sleeping space ≥2, AOR = 13.59, 95% CI (4.40–41.93) were found to be significantly associated with under-5 years old children ITNs utilization.ConclusionEven though, a significant proportions of under-5 years old children had access to ITN, only one-third of the participant child utilized it properly.