This　paper　aims　to　investigate　the　role　that　hierarchical　structure　plays　in　photocatalytic　process.　Hierarchical　rutile　TiO2　microspheres　assembled　by　nanowires　bundles　were　initially　synthesized　and　then　treated　by　thermal　annealing　or　ultrasonic　irradiation.　Various　technologies　including　SEM,　XRD,　DRS,　and　N2　sorption　were　employed　to　investigate　the　effect　of　post-treatments　on　the　microstructures　of　TiO2　samples.　The　results　revealed　that　annealing　treatment　caused　obvious　growth　of　particle　size,　decreased　surface　area　and　pore　volume,　and　weakened　optical　absorbance,　while　ultrasonic　irradiation　had　little　influence　on　these　properties.　However,　both　annealing　treatment　and　ultrasonic　irradiation　seriously　destroyed　the　microcavity　structure　which　existed　in　between　the　roughly　parallel　nanowires　of　hierarchical　TiO2　microspheres.　The　existence　of　microcavity　in　these　TiO2　hierarchical　microspheres　was　proved　to　be　crucial　to　the　photoactivity　toward　degradation　of　methyl　orange　(MO).　It　has　been　proposed　that　the　microcavities　could　act　as　microcapacitors　for　electronic　storage　and　therefore　contribute　to　a　high　separation　efficiency　of　photogenerated　electron-hole　pairs.　On　the　basis　of　ESR　and　active　species　scavenger　experiments,　h+　was　found　to　be　the　primary　active　species　in　MO　degradation　while　O2-　slightly　promoted　the　degradation　efficiency　via　the　formation　of　OH.　This　finding　is　of　great　significance　for　the　design　of　efficient　photocatalytic　materials　with　potential　applications　in　solving　worldwide　energy　crisis　and　environmental　pollution.　©　2014　Elsevier　B.V.