Background: Malaria can be prevented by using insecticide treated nets and decreases malaria mortality by 55% in under -five children in Africa. Ethiopia, scaling up distribution and utilization of insecticide treated nets to cover 100% of under –five children. Utilization of insecticide treated nets has many unsolved problem like, not being hung at all, poorly hung nets, and children not being prioritized to use. This study purpose was to assess insecticide treated nets utilization and its associated factors among under -five children in Lasta district. Methods: we conducted a community-based cross-sectional study among under-five children in Lasta district from April to May, 2022. Four study kebeles were identified by simple random sampling technique and 416 care takers with at least one under five children were selected by systematic random sampling technique. Semi-structured questionnaire was used to collect data using face-to-face interview. The data was entered into Epi data version 3.1 then exported to statistical package for social science version 25 for analysis. Pretest was conducted among 5% of sample. We used tables, frequencies, percentages, and means for descriptive analysis. Bivariable and multivariable logistic regression analysis were done. Variables with p-value <0.05 along with a 95% confidence interval (CI) were used to declared statistically significant variables. Hosmer-Lemeshow model was done. Results: A total of 403 under-five children were participated in the study with 95.72% response rate. Of these, 137 (34%) slept under insecticide treated net during the previous night prior to the survey. Care takers who had social support from others [(AOR: 1.74, 95% CI (1.10-2.74)], who had treated net [(AOR: 3.04, 95% CI (1.07-8.63)], who had good knowledge about insecticide treated net [(AOR: 10.01, 95% CI (5.37-18.69)], Care takers who think the ITN chemical is harmful [(AOR: 0.62, 95% CI (0.39-0.99)] were significantly associated with under five children insecticide treated nets utilization. Conclusions: Even if all have access to insecticide treated nets, only one third of the participant were utilized it. Care takers who had no social support from others, had non treated nets, had poor knowledge about insecticide treated nets and care takers who did not think insecticide treated net chemical is harmful were significantly associated factors. Thus, health education and awareness creation should be taken for those care takers to improve insecticide treated nets utilization for under-five children. Keywords: under-five children, insecticide-treated nets, utilization, malaria, Last district