Shear localization is an important failure mechanism in crystalline solid materials. However, while nanograins (NGs) are universally observed in shear bands, their connection with the formation of the bands is not clear yet. Based on molecular dynamics (MD) simulations, we propose that the shear localization evolves as a percolative process of deformation zones between NGs. The presence of NGs is a necessary condition to relieve some of the elastic energy by dislocation emission and absorption. Moreover, the density and spatial arrangement of those grains are of prime importance to accelerate or delay the formation of a strong localization path.