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Introduction: Unfortunately, today the incidence of abnormal body mass index (BMI) in adolescents is increasing for various reasons that can have a negative impact on their health. This study aimed to determine the status of BMI in adolescents and its socio-demographic predictors in adolescents was conducted in the city of Sanandaj in 1391. Materials & methods: This cross-sectional study was performed on 1,500 adolescents 18-14 years (750 boys and 750 girls) enrolled in public, semi-public and private schools in the city of Sanandaj. The sampling method was random. Multinomial logistic regression was used to determine socio-demographic predictors of BMI. Findings: The mean (SD) of BMI in adolescents was 20.89 (3.73) kg/m2. About a quarter of adolescents had abnormal BMI (4.9% and 21.2% respectively for lower and higher than the normal range). Gender, employment status, and use drug continually were predictors of abnormal BMI. The lower and higher range of BMI was more in boys. Frequency of higher rang of BMI was significantly more frequent in adolescent used drug continually and lower in adolescents who were working with education. Discussion & Conclusion: According to our results, the prevalence of abnormal BMI, particularly overweight and obesity is high in adolescents. It seems that further researches to determine the main reasons of this problem and practical solutions to solve it are urgent necessity for the health of adolescents and so the next generation.