Fibrous　TiO2　and　plate-like　TiO2　were　obtained　through　the　hydrothermal　synthesis　method　by　using　two　kinds　of　protonic　tetratitanate　(H2Ti4O9),　prepared　by　ion　exchange　of　K2Ti4O9　and　HCl,　or　milled　K2Ti4O9　and　HCl,　respectively.　The　product　made　by　hydrothermal　treatment　of　H2Ti4O9　without　milling　in　water　consisted　of　anatase　TiO2　and　retained　the　fibrous　morphology　of　the　precursor　but　with　fine　crystals　attached　on　the　surface,　formed　by　the　in　situ　topotactic　transformation　reaction　and　dissolution　and　recrystallization.　On　the　other　hand,　TiO2　prepared　with　H2Ti4O9　obtained　through　ion　exchange　of　milled　K2Ti4O9　and　HCl　had　plate-like　shape,　namely　retaining　the　morphology　of　nanosheets　of　H2Ti4O9　through　an　in　situ　transformation　process.　Under　ultraviolet　irradiation,　70%　methyl　orange　degradation　by　TiO2　nanosheets　was　about　3.3　times　higher　than　that　by　fibrous　TiO2.　The　higher　surface　area,　higher　pore　volume,　and　smaller　particle　size　led　to　the　higher　photocatalytic　activity　of　the　TiO2　nanosheets.