A pressure-induced phase transformation is used to refine the grain size of polycrystalline Y2O3, by a factor of 3000. A surface modification effect accompanies the observed grain refinement, which becomes more apparent with increasing holding time under high pressure. The surface-modified layer exhibits lower hardness and lower oxygen content relative to the underlying material. Moreover, it possesses columnar-grained structure with cubic symmetry, whereas the interior has a monoclinic structure.