Purpose: This study aimed to identify the effects of utilizing Smartphone Application Peer Support (SAPS) on health behavior and body mass index (BMI) among overweight or obese breast cancer survivors (BCS). Methods: A nonequivalent control group with a non-synchronized design was utilized and 36 participants (experimental group 14, control group 22) were recruited from August 2017 to September 2018. Participants were 40~65 years old, overweight or obese, had completed primary cancer treatment within the 12 months prior to the study, and had not done regular exercise during the last 6 months. The 3-month SAPS consisted of exercise and diet education (once p/2 weeks), peer support (once p/week), and self-monitoring using smartphone applications (5 times p/week). All participants underwent assessments at baseline, right after SAPS, and at 3 months after SAPS. Data were analyzed using repeated measures ANOVA. Results: At the completion of SAPS significant differences were found between groups in motivation for exercise (t=-3.24, p=.005), physical activity (t=-4.15, p<.001), total calorie intake (t=3.42, p=.002), calories from fat (t=-3.01, p=.005), intake of vegetables (t=-2.83, p=.008), and BMI (t=5.21, p<.001). Significant differences in BMI (t=4.13, p<.001) remained up to 3 months after SAPS completion. No significant differences was shown between groups in self-efficacy for exercise, either immediately after or 3 months after SAPS. Conclusion: The SAPS has the potential to improve motivation for exercise, health behavior, and BMI of BCS. However, special efforts are required to encourage participants to complete the intervention and maintain long-term effects for future trials.