Visible　light-activated　SnNb2O6　nanosheets　(NSs)　with　high　surface　area　and　small　crystallites　have　been　prepared　by　a　microwave-assisted　template-free　hydrothermal　method　without　exfoliation　for　the　first　time.　This　approach　could　be　used　to　prepare　the　functional　materials　efficiently　and　extended　to　synthesizing　two-dimensional　nanosheet　materials　directly　as　well.　The　crystalline　phases,　photoabsorption　performances,　and　surface　areas　and　porosity　of　the　samples　are　characterized　by　XRD,　UV-vis　diffuse　reflectance　spectroscopy　(UV-vis　DRS),　and　N-2-adsorption.　Results　show　that　a　hypsochromic　shift　of　the　photoabsorption　edge　is　observed,　which　reflects　an　obvious　quantum　size　effect.　TEM　images　reveal　SnNb2O6　nanosheets　with　a　thickness　of　1-4　nm　versus　several　hundred　nanometres　in　lateral　size.　Based　on　the　experimental　results,　the　formation　mechanism　of　SnNb2O6　nanosheets　is　also　studied　and　proposed,　which　reasonably　follows　a　synergy　interaction　of　reaction-crystallization　and　dissolution-recrystallization　processes.　Due　to　the　unique　morphology,　larger　surface　area,　smaller　crystallites　and　stronger　redox　ability　of　the　photogenerated　hole-electron　pair,　these　photocatalysts　show　much　higher　photocatalytic　activities　for　the　degradation　of　rhodamine　B　(RhB)　compared　with　their　counterparts　prepared　by　the　traditional　solid-state　reaction.　The　reaction　rate　is　enhanced　by　over　4　times　and　the　RhB　molecule　can　be　mineralized　into　CO2　and　H2O　over　SnNb2O6　NSs.　The　decomposition　mechanism　of　RhB　over　SnNb2O6　under　visible　light　irradiation　and　the　active　species　in　the　photocatalytic　process　have　also　been　discussed.