Purpose: The aim of this study was to test the applicability of the Transtheoretical Model (TTM) to gain an understanding of interdental cleaning behaviour change in senior high school students (12th grade) in Iran. Materials and Methods: Multistage cluster sampling was employed to recruit 361 senior high school students from 8 schools in Yazd City, Iran. This cross-sectional study took place from November 2008 to March 2009. Appropriate instruments were used to identify the stages of interdental cleaning behaviour and psychological attributes, including interdental cleaning behaviour, decisional balance, and self-efficacy. The statistical analysis of the data included descriptive statistics, t-test and ANOVA. Results: Of the 361 students, nearly 12.5% were in the maintenance stage, while 49.6% were in the pre-contemplation stage, with the rest distributed among the other stages of interdental cleaning behaviour change. There was a statistically significant difference in the stages of interdental cleaning behaviour change by gender. Self-efficacy and decisional balance differed significantly across the stages of interdental cleaning behaviour change. Conclusion: The TTM was found to be useful in determining the stages of interdental cleaning behaviour change among students. This study suggests the development of theory-based and empirically supported intervention strategies and programs to improve interdental cleaning behaviour with an emphasis on improving students' self-efficacy in this area. Keywords: decisional balance, interdental cleaning behaviours, transtheoretical model, self-efficacy, students