Coarse gamma ' precipitates with ordered structure (i.e. primary gamma ' precipitates) residing on grain boundaries in nickelbased superalloys are generally considered to be non-deformable, they are thus being tailored to restrict grain growth during hot deformation. We demonstrate here, using both transmission electron microscopy analysis and molecular dynamics simulation, that deformation actually occurred on these precipitates and it is dominated by 1/6 <112> Shockley partial dislocation shear mechanism. Molecular dynamics simulations further reveal that all the critical strains triggering plastic deformation of primary gamma ' under uniaxial tensile test are identically very low, depending on the deformation temperatures and strain rates. These findings provide new insight into the precise control of microstructure in nickel-based superalloy containing grain boundary precipitates.