The purpose of this research is to develop an immersive virtual fitness center as an intervention to promote physical activity and examine its impacts on users’ exercise plans and exercise behaviors. The virtual reality (VR)-based intervention includes three main features: enabling the users to control their avatar, personalizing the avatar to look the same as the user, and visualizing the positive consequences of exercising. We conducted an experiment to randomly assign participants to two treatment groups: experiencing either the self or another person losing weight because of exercising in the virtual environment. The findings demonstrated that the self-avatar group exercised more in the voluntary section than the other-avatar group. However, participants in the self-avatar condition perceived a lower level of sense of presence compared to participants in the other-avatar condition. Compared to people in the control condition who watched and followed the exercise from a workout video, those who exercised in the virtual fitness center, regardless of whether the avatar was based on the self’s image or another person’s image, planned to spend more time on exercising in the following week. Theoretical and practical implications for using VR technology to promote health-related behavioral change, and why personalization decreases perceived sense of presence in the virtual environment are discussed.