Background: Body mass index(BMI) is a measurement of nutritional status which is a vital precondition for good health. Globally, obesity and being overweight in children are growing problems that indicate high health risks in children's nutrition status. The main objective of this study is to investigate the factors associated with quantiles of under-five children’s BMI in Ethiopia. Methods: The information collected from 5323 children based on the Ethiopian mini Demographic Health Survey, 2019 dataset was considered in the study. The study variables like maternal socio demographic characteristics, child demographic characteristics, and behavioral factors were considered as determinants of the body mass index status of a child. The study used descriptive statistics and the Bayesian quantile regression model to identify significant factors that affect under-five children’s body mass index. Results: The study analyzed data from 5323 children and found that 5.09% were underweight (BMI <12.92), 10.05% were overweight (BMI: 17.06 – 18.27), and 5.02% were obese (BMI ≥18.2). The Bayesian quantile regression model revealed several significant factors affecting the BMI of under-five children. At lower quantiles, the current age of children was [95% CI :(-0.01, - 0.004)], maternal age [95% CI: (0.005, 0.02)], mother's primary education [95% CI: (0.08, 0.29)], secondary education or higher [95% CI (0.29, 0.58)], following Protestant [95% CI: (0.07, 0.37)], and being from the regions of Afar [95% CI: CI (-0.57, -0.08)] and Somalia [95% CI: (-0.96, -0.49)] were associated with BMI. Additionally, still breastfeeding [95% CI: (-0.41, - 0.10)], being female [95% CI: (-0.22, -0.06)], and wealth index [95% CI: (0.26, 0.61)] were also significant factors at upper quantiles. Conclusions: Currently the percentage of lower and higher BMI for under five children in Ethiopoia is high. The factors, current age of children, sex of children, the current age of a mother, the religion of the family follow, region and wealth index had a significant impact on BMI of under five children both at lower and upper quantile levels. Thus, administrators should plan and develop strategies to enhance the normal or healthy weight status of under-five children in Ethiopia. Keywords: Bayesian quantile, BMI, Under-five children, Ethiopia