The study investigated the roles of social support, self-efficacy, and behavioral change in physical activity (PA) in the Korean culture. The subjects were 164 Korean college students. In November 2016, the study participants completed an online survey asking about PA behavior, PA Self-Efficacy, PA Stages of Change (stages of behavioral change in PA), and Social Support for PA. The collected data were statistically analyzed through structural equation modeling. In the results, PA Stages of Change had a direct effect (β = 0.57, $p$ < .01) on PA with an explanatory power of $r^2$ = 0.46. Moreover, PA Self-Efficacy had an indirect (βindirect = 0.29) effect on PA through PA Stages of Change as PA Self-Efficacy had a direct effect (β = 0.50, $p$ < .001) on PA Stages of Change. However, Social Support for PA did not show an effect on other factors. Additionally, Social Support for PA had a correlation of $r$ = 0.45 ($p$ < .001) with PA Self-Efficacy. The study concluded PA Stages of Change is a significant PA predictor. Moreover, high PA Self-Efficacy indirectly helps Korean young adults to be more physically active by fostering advancement on the stages of behavioral change in PA. In contrast, Social Support for PA does not have a significant association with PA or PA Stages of Change. This non-significance of Social Support for PA can be explained with Korea's collectivistic culture. Furthermore, there is a possibility that Social Support for PA can positively affect PA by interacting with PA Self-Efficacy.