Commercial pure titanium sheet was rolled by means of asymmetric rolling in order to obtain nanocrystallines in the top-surface layer, the microstructural evolution was examined by using different experimental techniques. Experimental results show that, after the asymmetric rolling, equiaxed nanocrystallines of about 30-60 nm in size form in the top-surface layer of sheet. The research work indicates that the surface nanocrystallization can be realized for large-dimensional metal sheets during the rolling process by using the asymmetric rolling. The asymmetric rolling induced surface nanocrystallization mechanism was summarized as follows: upon the application of repeated loads, dislocation cells/sub-micro-grains form through slips, annihilations and recombinations of high density of dislocations; with the increment of rolling pass and reduction,high density of dislocations in the refined cells/grains developing in above route lead to the reduction of grain size and the increment of misorientations between the refined grains, and finally equiaxed nanocrystallines with medium to large angle grain boundaries form. A gradient-structured surface layer of about 30μm thick was observed to form in the initial stage,its thickness increases with increasing rolling reduction, and the hardness variation along the depth was found to relate to the grain size.