1Research has shown that mindfulness meditation not only improves our cognitive and 2 motivational functioning (e.g., attention, mental health), it influences the way how our brain 3 networks [e.g., default mode network (DMN), front-parietal network (FPN), and sensory-motor 4 network (SMN)] function and operate. However, surprisingly little attention has been paid to the 5 possibility that mindfulness meditation alters the structure (composition) of these functional brain 6 networks. Here, using a single-case experimental design with longitudinal intensive data, we 7 examined the effect of mindfulness mediation on intra-individual changes in the composition of 8 whole-brain networks. The results showed that mindfulness meditation (1) changed the community 9 size (with a number of regions in the FPN being merged into the DMN after mindfulness 10 meditation), (2) changed the brain regions composing the SMN community without changing its 11 size, and (3) led to instability in the community allegiance of the regions in the FPN. These results 12 suggest that, in addition to altering specific functional connectivity, mindfulness meditation leads to 13 reconfiguration of whole-brain network structure. The reconfiguration of community structure in the 14 brain provides fruitful information about the neural mechanisms of mindfulness meditation. 15 16 3 Author Summary 1Training to focus on present-moment experiences, i.e., mindfulness meditation, is known 2 to produce significant beneficial outcomes, such as improved attention and mental health. It is 3 important to understand how these beneficial effects are produced at the neural level; however, 4 previous research investigating this question almost exclusively employed a pre-post design (i.e. 5 assessing brain functioning only once or twice) or comparison between beginners and experts. The 6 current study takes a fundamentally different approach ----we repeatedly assess the brain function of 7 a single person over many days (intensive longitudinal design) and show that mindfulness mediation 8 significantly changes the size, composition, and variability of the brain networks that are related to 9 sensory processing, executive control, or spontaneous thoughts and self-referential processing. These 10 results highlight the importance of investigating intraindividual changes for understanding the neural 11 mechanisms of mindfulness meditation and suggest that such changes may be used as a biomarker to 12 reflect the efficacy and progress of mindfulness meditation. 13 14 15 1