BACKGROUND AND AIM: During the COVID-19 pandemic, an Internet-Mindfulness-Based Stress Reduction (iMBSR) program was delivered and may be better than an in-person approach. Our study evaluated the effects of iMBSR intervention on mental health, self-efficacy, and body image in women with breast cancer in Taiwan. MATERIALS AND METHODS: Sixty-seven women with breast cancer were allocated to a 6-week iMBSR ( = 41) program or a waitlist control group ( = 26), without heterogeneity between group characteristics. Patients from both groups were measured at baseline and postintervention using three scales: Depression, Anxiety, and Stress Scale (DASS-21), General self-efficacy scale, and Body Image Scale. Descriptive dataset analysis, paired -test, and Student's -test were used to evaluate the data. RESULTS: Although iMBSR did not significantly improve depression and stress between groups, iMBSR could improve anxiety (Δmean: -2.0 vs. -0.4,  = 0.041) with medium effect sizes. Significant benefits were found for body image (Δmean: -3.6 vs. 0.9,  = 0.003) and self-efficacy (Δmean: 4.2 vs. 1.5,  = 0.004), with large effect sizes (Cohen's  = 0.73). CONCLUSION: Our preliminary study supports iMBSR as a program that can improve mental health, body image, and self-efficacy in women with breast cancer. During the COVID-19 pandemic, medical professionals can use Internet-based clinical health education.