H 2 Ti 4 O 9 nanocrystals with high specific surface areas were prepared by delamination and reassembly process through a way of ball milling combined with ion exchange reaction. The samples were characterized by X-ray powder diffraction, transmission electron microscopy, chemical analysis, thermal analysis, N 2 adsorption-desorption isotherm, and absorption spectrum. The crystallites of Ti 4 O 9 2− in the form of titania nanosheets has lateral size less than 50nm. The specific surface area of H 2 Ti 4 O 9 nanocrystals depends on pH value of reassembling solution and ball milling time. H 2 Ti 4 O 9 nanocrystals prepared by ball-milling of K 2 Ti 4 O 9 for 2h and suspending in 1M HCl followed by precipitation at pH 8 had the specific surface area of 328.4m 2 g −1 which was about 16 times larger than the fibrous H 2 Ti 4 O 9 prepared by treating original K 2 Ti 4 O 9 without ball-milling in 1M HCl.