Crack　dynamics　typically　controls　the　deformation　and　fracture　of　nanocrystalline　(NC)　metallic　materials,　while　the　ubiquitous　interplay　between　nanograins　and　propagating　cracks　remains　unclear.　Here,　using　in　situ　nanomechanical　testing　inside　transmission　electron　microscope,　we　clarify　the　atomistic　interplay　between　GB　evolution　and　crack　propagation　in　NC　silver　(Ag)　thin　films,　where　the　cooperative　GB　migration,　GB　re-orientation　and　nanograin　nucleation　activated　ahead　of　the　crack　tip　result　in　conspicuous　crack　blunt.　This　grain-crack　interplay　is　critically　facilitated　by　the　diffusive　plasticity　associated　with　crack　propagation,　which　yields　dynamically　modified　crack　tip　morphology　and　rate-limiting　intragranular　cracking.　These　atomistic　mechanisms　of　diffusive　crack-grain　interplay　enrich　our　understanding　of　the　cooperative　deformation　behaviors　in　nanograin　regime　and　hold　general　implications　to　enhance　the　fracture　resistance　of　NC　metallic　materials.　©　2021