Low mobility is the 4th common cause of human death. The Trans-Theoretical Model (TTM) is a health behavior change theory that describes the process of how people change their behavior. This study was conucted with the aim of investigating the determinants of Physical Activity (PA) behavior based on the TTM in 30-50-year-old individuals. In this cross sectional descriptive-analytical study, 240 individuals aged 30-50 years old in Delijan, Iran were randomly selected based on family case records in comprehensive health centers. The data were collected by a valid and reliable questionnaire, which consisted of demographic information and questions related to the TTM structure. The data were analyzed by SPSS 22 software, using Pearson correlation coefficient and linear regression. 78.3% of the participants were in tht stages of pre-contemplation, contemplation, and preparation of PA. The stages of change were the best predictor of PA (p<0.001; β=0.45), which could explain 29% of variance of PA. Also, self-efficacy (p<0.001; β=0.51) and change processes (p<0.001; β=0.26) predicted the stages of changes in PA and could explain 32% of variance of stages of changes in PA. The stages of change, considering self-efficacy and change processes, are predictors of PA behaviors.