Introduction: Some inter-proximal areas of teeth are not available to brushing thus dental flossing for eliminating plaque is needed. The aim of this study was to assess the determining factors based on trans-theoretical model (TTM) among high school students. Methods: This descriptive, analytical, cross- sectional study was conducted during April and May 2012. 653 high schools students of Pakdasht in Iran were selected through multi-stages cluster-sampling method and participated in this study. A Researcher-designed questionnaire, including demographic information, stage of change, self efficacy, perceived benefits and barriers parts for collecting data was used.Data were collected based on TTM framework. Content validity of the questionnaire was determined using Lawsche's formula and CVI=0.97 and its reliability was specified through Cronbach's alpha coefficient (0.83). Data were then analyzed by SPSS software V15. Logistic Regression, spearman Correlation, ANOVA and nonparametric tests were used as the statistical methods. Results:The majority of students 73% (n=477) in dental flossing were in pre-contemplation and contemplation stages of change. Self efficacy and perceived benefits scores were lower in primary stage of change and perceived barriers were higher at the primary stage (p<0.000). Moreover, it was found that increase in perceived self efficacy enhances chance of dental flossing and maintaining this behavior more than six month, and increase in perceived barriers of people declines this chance (p<0.000). Conclusion:The findings showed a high percentage of students were in pre-contemplation and contemplation stages in dental flossing behaviorthus probably will expose with inter-dental decays in the future. Self efficacy, perceived benefits and barriers are determinant factors of dental flossing which should be considered in educational programs. Keywords: Dental floss, Educational model, Students