Identification of the behavioural, normative and control beliefs influencing children's behaviour is an important prerequisite in designing effective interventions. The current study aims to evaluate the effectiveness of an intervention program, based on the Theory of Planned Behaviour (TPB), on obesity indices and blood pressure (BP) in Ioannina, Greece.Participants were 646 fifth grade pupils (360 girls and 286 boys). The intervention group (IG) consisted of 321 children in 13 randomly selected schools while the rest (n = 325 children) served as the control group (CG). Based on the outcome of the TPB questionnaire, the intervention focused on overcoming the barriers in accessing physical activity areas, increasing the availability of fruits and vegetables and increasing parental support. General linear mixed model and mediating variable analysis were used to evaluate the differences between the two groups and to test whether changes in certain dietary, physical activity and anthropometrical indices mediated the effect of the intervention on BP.IG had higher consumption of fruits and lower consumption of fats/oils and sweets/beverages compared with the CG. Intervention's effect on BMI could be explained by the changes in fruit and fats/oils intake whereas the reduction of systolic and diastolic BP could be explained by the reduction of BMI.The findings indicate favourable changes in BP and obesity indices after the implementation of a 1-year school-based intervention program based on the TPB. These results highlight the importance of developing a social and physical environment that promotes balanced eating behaviours and extra-curricular access to physical activity venues.