Purpose - The purpose of this paper is to determine the effectiveness of a modified HBM-based intervention to reduce body mass index (BMI) for age in overweight junior high school students.

Design/methodology/approach – A cluster-randomized controlled trial was conducted in the first and second of a junior high school in the center of Thailand. In total, 24 classrooms were randomly assigned to a modified health belief model intervention arm (HBMIA), and 24 classrooms were randomly assigned to a traditional school health education arm (control). In total, 479 students who were overweight (BMI forage = median +1SD, age 12–15 years) participated in the study. The HBMIA used the health belief model (HBM) as a motivator for behavior strategies that included modifying diet and participating in physical activity. BMI, health knowledge and behavior for preventing obesity were recorded at baseline and at six months. A multilevel regression model was performed to calculate mean difference between HBMIA and control group.

Findings – The students who participated in the HBMIA showed a decrease in BMI of 1.76 Kg/m², while those who participated in the control showed an increase in BMI of 1.13 kg/m², with a mean difference of -2.88 kg/m² (95% CI = -3.01 to -2.75), an improvement in health knowledge (mean difference 23.54;95% CI 22.60-24.48).

Originality/value – A modified HBM-base intervention to reduce BMI for age is effective in overweight junior high school students.