High strain rate superplasticity was obtained for a powder metallurgy pure aluminum, 1N90. The material exhibited the grain sizes of 15.2 mum with the subgrains of 1 mum before superplastic deformation. The microstructure of the material did not change much after superplastic deformation. Present result indicates that the subgrains with the misorientation angle >5degrees play an important role through boundary sliding during superplastic deformation. The optimum stain rate was depended on subgrain size with misorientation angles more than 5degrees, not grain size. This is a new phenomenon which never reported before. (C) 2002 Elsevier Science B.V. All rights reserved.