Objective: Regular physical activity plays an important role as a protective factor in the treatment of coronary artery disease. Therefore, understanding the psychosocial factors that are involved in the process of changing and maintaining physical activity in the long term is fundamental in supporting patients’ lifestyle changes. The Health Action Process Approach (HAPA) health behavior model provides a practical, theoretically grounded, and empirically validated conceptual framework for understanding and predicting these factors. The present study aims to explore the factors involved in the process of physical activity of persons with coronary artery disease. Methods: The present longitudinal study included 117 patients with coronary artery disease (n = 117; 77 men, 40 women; mean age = 62.48 ± 6.22) who underwent CABG or PCI for the first time. At three measurement points, we examined the factors of the HAPA model and the physical activity of the patients. The test the direct and indirect associations of the variables structural equations modeling with latent variables was applied. Results: In the formation of behavioral intention the positive outcome expectancies proved to be an important factor. In translating intention into behavior, the constructs of planning, self-efficacy regarding the actual behavior, self-monitoring, and social support seemed to have significant roles. Together, the variables included in the present model explained 41% of the variance of physical activity. Conclusions: The results draw attention to the role of expectations about the positive consequences of physical activity as well as the importance of planning, self-efficacy, self-monitoring, and social support, which thus can be important factors in supporting patients with coronary artery disease to change their exercise habits in the long run. By incorporating these factors into the psychosocial intervention of patients, it is reasonable to expect that regular exercise habits formed during cardiological rehabilitation can persist in the long term.