MBSR(BC) is known to have a positive impact on psychological and physical symptoms among breast cancer survivors (BCS). However, the cognitive mechanisms of how MBSR(BC) works are unknown. The purpose of this study, as part of a larger R01 trial, was to test whether positive effects achieved from the MBSR(BC) program were mediated through changes in increased mindfulness, decreased fear of breast cancer recurrence, and perceived stress. Female BCS >21 years diagnosed with Stage 0-III breast cancer were randomly assigned to a 6-week MBSR(BC) or a Usual Care(UC) regimen. Potential outcome mediators were identified by use of an analysis of covariance (ANCOVA), comparing mean values of outcome variables and potential mediating variables followed by mediational and bootstrap analyses. Among 322 BCS (167 MBSR(BC) and 155 UC), fear of recurrence and perceived stress, but not mindfulness, mediated reductions in anxiety and fatigue at weeks 6 and 12, partially supporting our hypothesis of cognitive mechanisms of MBSR(BC).