Background: Theory-oriented interventions play a key role in promoting people's QOL and preventing the risk of immobility. Given this key role and due to the critical nature of nurses' professional duties, this study was performed. Objective: The study aimed to determine the effect of a TTM-based intervention on level of physical activity in ICU nurses working at hospitals affiliated to Guilan University of Medical Sciences. Methods: This randomized controlled trial first selected 82 nurses through the census method in order to determine their SOC, then assigned 68 nurses who were in the first three stages to case and control groups. Valid and reliable data collection tools were used, including a demographic questionnaire and TTM constructs (SOC, GPAQ, SE, DB, and POC). Path analysis of data was performed by Lisrel-8.80. The inter-correlation matrix was determined within the model's constructs and between the constructs and the level of physical activity in order to design and implement the intervention. Two-stage assessment was performed, once immediately after the intervention and again six months later. The data obtained were analyzed using appropriate statistical tests and Pearson correlation coefficient. Results: 82.9% of the participants were in the first three stages of TTM. Immediately after the intervention, 55.9% were in the preparation stage, 44.1% in the pre-contemplation stage, and 76.5% in the contemplation stage. Before the intervention, only 26.5% were in the preparation and 44.1% in pre-contemplation stages. There was a statistically significant difference (P = 0.0001) between the control and case groups immediately and six months after the intervention. Conclusion: TTM-based intervention led to the subjects' placement in higher levels of SOC.