Background & Objectives: Sedentary lifestyle is one of the most important modifiable factors associated with diabetes when the level of physical activity of the diabetics is poor. This study aimed to determine the predictive power of Trans Theoretical Model (TTM) for physical activity (PA) behavior in diabetic patients. Methods: In a cross sectional study 393 patients with type 2 diabetes in Khorramabad city were selected by convenience sampling method in 2013. Data were collected through a questionnaire consisted of 5 sections and analyzed by Pearson correlation coefficient, chi-square, t-test, ANOVA and multiple linear regression tests using SPSS-18 software. Results: In this study the majority of the participants (48.9%, 192 patients) were in pre contemplation stage and only 15% and 6.6% were in the action and maintenance stages, respectively. A significant difference was seen in stages of behavior change among educational levels. Mean daily PA in maintenance group was significantly higher than the other groups. A negative significant relationship was found between the average daily PA and perceived benefits and positive significant correlations were found between the average daily PA and processes of change, self-efficacy and decision balance. Multiple regressions showed that 33.8% of daily PA variance can be explained and predicted by TTM constructs. Conclusion: Stages of change theory had appropriate predictive power for PA levels in diabetic patients. Paying attention to roles of this theory structures on promoting PA as well as the providing related educations in diabetes associations and health centers are recommended.