Local thickening

resulting from displacive or diffusive mechanisms individually

is frequently observed in nanoscale metals during compression. However

it is not fully understood how a coupled displacive-diffusive mechanism would mediate the compressive deformation of metallic nanocrystal. Here

by conducting

in situ

atomic-scale compression tests

we find that

instead of local thickening commonly reported thus far

abnormal thinning events consecutively occur in silver nanocrystals

where preexisting dislocations and crystal slip serve as stimuli to activate surface atom diffusion. Below a critical width

tensile-fracture-like failure occurs in the silver nanocrystals. This work sheds light on atomic-scale diffusion-mediated compressive deformation of metallic nanocrystal.