Previous　molecular　simulations　and　experiments　suggest　that　grain　boundaries　(GBs)　serve　as　dislocation　sources　in　nanocrystalline　metals.　Although　a　large　number　of　studies　have　been　carried　out,　direct　experimental　evidence　of　dislocation　nucleation　from　GBs　has　rarely　been　achieved.　In　this　work,　we　performed　in　situ　transmission　electron　microscopy　(TEM)　observations　for　a　Pt　nanocrystalline　film　with　an　average　grain　size　of　-10　nm　during　tensile　deformation.　This　study　revealed　direct　evidence　of　dislocation　nucleation　at　the　GBs　at　the　atomic　scale.　This　is　different　from　the　common　hypothesis　predicted　by　molecular　dynamic　simulations　that　only　partial　dislocations　are　emitted　from　GBs　in　small-grained　structures.