Anatase　TiO2　mesocrystals　and　single　crystals　with　dominant　{101}　facets　were　successfully　synthesized　without　any　additives　using　titanate　nanowires　as　precursors　under　solvothermal　and　hydrothermal　conditions,　respectively.　It　is　proposed　that　the　oriented　self-assembly　process　for　the　formation　of　TiO2　mesocrystals　was　controlled　by　the　same　thermodynamic　principle　as　that　of　single　crystals　in　this　simple　reaction　system.　Furthermore,　the　TiO2　mesocrystals　were　applied　in　photoelectrochemical　(PEC)　water　splitting　and　demonstrated　much　enhanced　photocurrent,　almost　191%　and　274%　compared　with　that　of　TiO2　single　crystals　and　commercial　P25,　respectively.　Electrochemical　impedance　measurements　under　illumination　revealed　that　the　photocurrent　increase　was　largely　ascribed　to　the　effective　charge　separation　of　electron-hole　pairs　and　fast　interfacial　charge　transfer.　This　could　be　attributed　to　the　intrinsic　characteristics　of　the　mesostructured　TiO2　composed　of　highly　oriented　nanocrystal　subunits　offering　few　grain　boundaries,　nanoporous　nature　and　a　short　transport　distance.