Aim: The study was conducted to determine the prevalence of obesity and the effective risk factors in adolescents in the age group of 15-18 years at a secondary school located in the province of Çankırı.  Methods: This descriptive and cross-sectional study was conducted with a total of 402 adolescents in the age group of 15-18 years studying at a secondary school located in the province of Çankırı between February and April 2017. Body weights and heights of the adolescents were measured, and their body mass indexes were calculated.  Results: It was determined in the study that 16.7% of the students were obese. The prevalence of obesity was found to be associated with a high socio-economic level, the long sedentary period in front of television and computer, decreased physical activity, family history of obesity, fast eating, and diet (p