H2Ti4O9 nanocrystals with high specific surface areas were prepared by exfoliation and reassembly process through a novel way of ball milling and ion-exchange reaction. The specific surface area of H2Ti4O9 nanocrystals depends on solution pH value and ball milling time. H2Ti4O9 nanocrystals precipitated by milling K2Ti4O9 for 2h and exfoliating in 1 M HCl followed by adjusting solution pH at 8 has the specific surface area of 328.2 m(2)/g.