H2Ti4O9　nanocrystals　with　high　specific　surface　areas　were　prepared　by　delamination　and　precipitation　process　through　ball　milling　combined　with　ion　exchange　reaction.　The　samples　were　characterized　by　X-ray　powder　diffraction,　transmission　electron　microscope,　thermal　analysis,　N-2　adsorption-desorption　isotherm,　and　absorption　spectrum.　The　crystallites　of　Ti4O92-　in　the　form　of　titania　nanosheets　have　lateral　size　less　than　50nm.　The　specific　surface　area　of　H2Ti4O9　nanocrystals　depends　on　pH　values　of　precipitation　solution　and　ball　milling　time.　The　specific　surface　area　of　H2Ti4O9　nanocrystals　prepared　by　ball-milling　of　K2Ti4O9　for　2h　and　suspending　in　1mol/L　HCl　followed　by　precipitation　at　pH=8　can　reach　328.4m(2)center　dot　g(-1).