A　series　of　TiO2　photocatalyst　with　well　mesoporous　structures　and　high　crystallinities　were　rapidly　fabricated　by　ultrasonic　irradiation　at　room　temperature.　The　prepared　samples　were　characterized　by　physical　adsorption　of　N2,　X-ray　di_raction　(XRD),　scanning　electron　microscopy　(SEM),　and　transmission　electron　microscopy　(TEM).　The　effects　of　the　power　and　time　of　the　ultrasonic　irradiation　on　the　crystallinity,　surface　area,　pore　volume　and　pore　size　of　the　TiO2　were　investigated.　The　formation　mechanism　for　the　mesoporous　TiO2　photocatalyst　under　ultrasonic　irradiation　was　proposed.　The　photocatalytic　activity　of　the　samples　was　evaluated　by　photocatalytic　degradation　of　methylic　orange　under　UV　light　(λ=365　nm)　irradiation.　The　results　showed　that　the　increase　of　ultrasonic　irradiation　power　will　increase　the　crystallinity,　pore　size　and　dispersion　of　TiO2　and　decrease　the　size　of　the　grain.　A　high　photocatalytic　activity　was　obtained　in　the　ultrasonic　fabricated　samples,　which　could　be　attributed　to　the　high　crystallinity　and　high　surface　area　and　pore　volume　of　the　photocatalysts.