Using a newly developed nanoscale deformation device, atomic scale and time-resolved dislocation dynamics have been captured in situ under a transmission electron microscope during the deformation of a Pt ultrathin film with truly nanometer grains (diameter d< ~ 10 nm). We demonstrate that dislocations are highly active even in such tiny grains. For the larger grains (d ~ 10 nm), full dislocations dominate and their evolution sometimes leads to the formation, destruction, and reformation of Lomer locks. In smaller grains, partial dislocations generating stacking faults are prevalent.