A　detailed　monitoring　of　atomic-scale　processes　accomplishing　grain　rotation　is　important　for　understanding　the　deformation　mechanisms　in　nanocrystalline　metals.　However,　direct　observations　have　been　rare　thus　far,　such　that　our　knowledge　about　grain　rotation　has　to　rely　heavily　on　hypothetical　models　and　simulations.　Here,　we　present　in　situ　atomic-resolution　evidence,　indicating　that　the　atomic　processes　accomplishing　grain　rotation　in　nanocrystalline　Pt　depend　on　the　type　of　grain　boundary　(GB)　separating　the　rotating　grains.　The　grains　around　general　(mixed)　GBs　rotate　via　the　Frank-Bilby　dislocation　activities　together　with　atomic　shuffling　and　disconnection　activities,　the　former　being　the　generation,　climb,　glide　and　reaction　of　GB　dislocations,　whereas　the　latter　leading　to　GB　migration　accompanying　the　grain　rotation.　While　for　the　grains　with　a　tilt　GB　in　between:　their　rotation　is　accomplished　almost　entirely　via　the　Frank-Bilby　dislocation　activities.　We　also　discover　that　the　GB　dislocation　climb,　glide,　and　reaction　often　involve　the　formation　and　destruction　of　Lomer-like　dislocations.　(c)　2022　The　Authors.　Published　by　Elsevier　Ltd　on　behalf　of　Acta　Materialia　Inc.