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.