Abstract: A nanostructured surface layer was fabricated on a pure Ti plate by surface mechanical attrition treatment (SMAT). The microstructure and thermal stability of the nanocrystallized layer annealed at different temperatures for various times were characterized. It is shown that the coarse-grained structure in the surface layer was refined into equiaxed ultrafine grains (about 12 nm) with isometric and random crystallographic orientations after SMAT. When the annealing temperature was lower than 773 K, little change in grain size was found, indicating satisfactory thermostability. The specimen annealed at 773 K for 150 min showed a slight recrystallization in the surface layer. However, the grain size of the sample increased obviously when it was annealed at 773 K for 240 min, and the value of the surface layer's hardness along the depth became smaller than that of the un-annealed SMAT specimens, suggesting the excellent thermostability disappeared.