Grain boundary diffusion creep equations developed previously for nanocrystalline materials were reappraised in order to elicit further understanding of plastic deformation of these materials in relation to grain boundary diffusion. From a mechanistic viewpoint, the strain rate is inversely proportional to the second power of the grain size when the grain size is refined to the same order of the grain boundary thickness. The presence of the threshold stress appears to be inherent, as a relatively large volume fraction of the grain boundary region is associated with irregularities.