Background Health volunteers are women who have humanistic cooperation with healthcare centers to prevent diseases. Health volunteers play an important role in the community. Thus if educated, they could be of great help to health care systems. Since physical inactivity and sedentary lifestyles increase the risk of all cause mortality, the aim of this study was to educate health volunteers on physical activity (PA) applying the theory of planned behavior (TPB). Methods This was a randomized controlled trial of an educational intervention to improve physical activity among health volunteers in Shushtar (a southern city), Iran in 2016. A sample of female health volunteers was entered into the study and randomly was assigned to the intervention and the control groups. The intervention included three educational sessions and a walking program. A questionnaire containing items on TPB constructs including: attitude, subjective norms, perceived behavioral control, intention and behavior constructs, the international physical activity questionnaire, and the physiological cost index (PCI- an objective measure of PA) were used to collect data at baseline and 6 months follow-up assessments. Then the data were compared within and between groups using t-test and paried test. Results In all 110 ‘Health Volunteers’ (55 per each group) were entered into the study. The mean age of participants was 35.65 ± 10.25 years. The results obtained from analysis showed that educational program improved attitude towards PA, perceived behavioral control, intention, behavior and PCI among the intervention group (P < 0.001). However, no significant changes occurred in the control group in terms of the study variables (P > 0.05). Additionally, comparison of score changes between two groups showed significant differences for all variables except for subjective norms and physical activity behavior (minute per day) group (P > 0.05). Conclusion The TPB-directed educational program improved physical activity and physiological cost index. The study framework might be used as a practical template for interventions aimed at improving physical activity among women.