Background & objectives: Trans-theoretical Model is one of theoretical approaches to the promotion of physical activity which represents behavior change as a process which passes through the definite stages. This study was conducted to investigate application of Trans-theoretical Model to identify determinants of physical activity behavior in the college students. Methods: This cross-sectional study was performed with participation of 200 college students which selected by systematic random sampling method. Data gathering instrument was a Questionnaire that includes: Stages of Exercise Change Questionnaire (SECQ), self-efficacy for physical activity and decisional balance questions. Test-retest method was used for reliability of SECQ and Alpha Cronbach's Methods for self-efficacy and decisional balance questions (0.82, 0.72, and 0.77, respectively). Data was analyzed using SPSS19 by use of central indexes, one way ANOVA, chi-square, Pearson correlation coefficient and regression. Results: Generally, distribution of individuals according to the physical activity stages was as follow: pre-contemplation 42%, contemplation 13%, preparation 15%, action 11.5%, and maintenance 18.5%. Average physical activity in week was 150.6±161.2 minutes. The level of physical activity through the stages of change (from precontemplation to maintenance) showed an increasing trend (p<0.001). Stages of change, self-efficacy and decisional balance, significantly predicted physical activity (p<0.001). Conclusion: Given to the importance of the Trans-theoretical Model constructs in predicting physical activity behavior, we propose that educational intervention based on these constructs, performed to promote physical activity, especially for the students of passive stages (pre-contemplation, contemplation and preparation).