Purpose: To determine the impact of education on stages of change of behaviour in Iranian senior high school students for interdental cleaning based on the transtheoretical model. Materials and Methods: This experimental study took place from April to November 2010. 306 students were selected by multistage cluster sampling and placed into two groups: control (153 students) and intervention (153 students). Appropriate instruments and the intervention programme were designed with the purpose of improving stages of interdental cleaning behaviour, perceived benefits and self-efficacy, as well as reducing perceived barriers and gingival index (GI). The impact of the intervention programme was assessed after 24 weeks and the GI of each student was recorded for both groups before and after intervention. The data were analysed using SPSS software and the chi-square, t test, ANOVA, paired t test, Mann-Whitney U-test and sign test. Results: The intervention had a significant, positive impact on improvement of the stages of interdental cleaning behaviour, increase in self-efficacy, perceived benefits, decrease in perceived barriers and improvement of GI (P 0.001). After the intervention, the average grades of self-efficacy, perceived barriers and perceived benefits in the control group vs the intervention group were significantly different (P 0.001 to 0.01). Conclusion: The programme was found to positively influence the stages of change and potential indicators of interdental cleaning behaviour and GI. It is suggested that this model be used for interventions in the other population groups. Schlagwörter: gingival index, oral and dental self-care behaviour, transtheoretical model