Background and Objective: Increasing the consumption of fast foods among adolescents is one of the challenges for planners and policymakers in the public health field. Identifying behavioral patterns and using appropriate interventions is a way to influence and change these patterns. Aiming to understand the status of fast food consumption, this study was conducted between secondary school students of Yasouj city. Materials and Methods: This is a formative research that used a sequential mixed-methods approach to collect and analyse the data. To collect the data, in quantitative stage, knowledge, attitude, and behavior (KAB) questionnaire, that utilised in social marketing, has been used. In the qualitative stage focused group discussions have been used. For mixed-analysis in both of quantitative and qualitative stages, respectively, the methods of correlation analysis and qualitative content analysis was done using SPSS 22 and MAXQDA 2018. Results: In the quantitative stage, in addition to identify demographic, attitude and behavioral characteristics, the relationship between this variables and fast foods consumption has been determined. These findings indicated a significant relationship between gender, knowledge, attitudes, and mother's education and occupation with fast foods consumption. In the qualitative section, five themes and 20 subthemes were emerged. Five main themes included: Barriers and Motivators, the role of parents, the role of the school's authorities, as well as distribution channels. Conclusion: Through formative research, we can design and implement more effective interventions to health promotion. These findings provide valuable insights for policymakers and social marketers to change the nutritional patterns of students in in down-stream area.