Introduction: Adolescence is the most important period of life where growth and development are accompanied, leading to increased demand for nutrients which could pose a greater risk of under nutrition. Little emphasis is given to reveal the evidence of level of under nutrition in this age group. Objective: the objective of this study was to assess the level of under nutrition and associated factors among adolescent girls in Soddo Dachi woreda ,South West Shewa. Methods: School based cross sectional study was conducted from January to February 2017, in Soddo Dachi Woreda. The data was collected by using interviewer administered questionnaire and Anthropometry. The study was conducted among 426 second cycle primary and secondary schools (5th-10th) grade adolescent girls. The WHO 2007 growth reference was used as a standard reference for classifying level of under nutrition of adolescents using WHO Anthroplus software version 1.0.4. All statistical analyses was done using the SPSS version 22 . Logistic regression model was used to identify the relationship between independent and outcome variables. The degree of association between dependent and independent variables were assessed using odds ratio with 95 % confidence interval and variables with p-value less than 0.05 were considered significant. Major findings: The prevalence of Thinness is 14.2%( 95% C.I.) and the prevalence of stunting is 7.2% (95 % C.I) source of drinking water and Menarche are associated with under nutrition. Conclusion: The prevalence of stunting is relatively low and the prevalence of thinness is medium when compared to the WHO classification of severity of malnutrition by prevalence among adolescent girls in Soddo Dachi woreda,South West Shewa. Recommendations: Sources of drinking water should be improved and growing vegetables and fruits should be encouraged.