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. Display omitted •The atomic-scale plastic deformation mechanism in a nano Pt film with a mean grain size of 10 nm were studied by in situ TEM.•The dislocation nucleation at grain boundaries was directly revealed at the atomic scale.•The dislocations were identified to be full dislocations.