Oat　bran　was　fermented　by　filamentous　fungi　to　improve　the　extractability　of　β-glucan.　Box-Behnken　experimental　design　and　response　surface　methodology　were　used　to　obtain　the　maximum　extractability　of　β-glucan.　Inoculum　volume,　fermentation　temperature,　and　time　were　evaluated　as　three　variables.　Under　optimal　fermentation　conditions,　the　extractability　of　β-glucan　in　oat　bran　increased　to　45.57　±　1.82%　and　51.10　±　2.32%　by　Aspergillus　niger　and　Rhizopus　oryzae,　respectively.　The　extractability　was　about　3-fold　of　that　before　fermentation　(16.86　±　0.76%).　Fermentation　of　oat　bran　by　Aspergillus　niger　and　Rhizopus　oryzae　reduced　the　molecular　weight　of　the　extracted　β-glucan　from　6.74　×　105　to　2.84　×　105　and　2.20　×　105　Da,　respectively.　Correspondingly,　the　apparent　viscosity　of　the　extracted　β-glucan　decreased　after　fermentation.　But　the　increased　extractability　compensated　for　the　reduced　molecular　weight　in　the　formation　of　viscous　solution.　The　molecular　structure　and　cellotriosyl/cellotetraosyl　ratio　were　not　affected　by　filamentous　fungi　fermentation.　©　2018　Elsevier　Ltd