Background: Malnutrition is the most serious human health and social problems that affect vast population of the world. Malnutrition continues to be a major public health problem throughout the developing world, particularly in southern Asia and sub-Saharan Africa. Adolescents from developing countries are more vulnerable to nutritional deficiencies due to early childhood nutritional problems, which include underweight, stunting and low dietary intakes Objective: To assess determinants of under nutrition among adolescents in Burayu woreda, Oromia Region, May 2011. Methods and materials: Community based unmatched case-control study design was conducted in Burayu woreda, Oromia Region from 10-25 March 2011. Cases comprised undernourished adolescents aged 10-19 years and controls were well-nourished adolescents. The study subjects were selected with probability proportional to size from the six kebeles. Using simple random sampling technique data was collected from 91 cases and 91 controls .Weight was measured in kilogram using digital bath room weighing scale to the nearest 0.1kg and height was measured in centimeter using a portable measuring board to the nearest 0.1 cm. Data was e