Oat　β-glucan　is　a　linear　polysaccharide　composed　of　D-glucopyranosyl　residues　arranged　as　mixtures　of　consecutively　β-(1→4)-linked　glucose　units　in　blocks　that　are　separated　by　single　β-(1→3)　linkages.　As　a　soluble　dietary　fiber,　oat　β-glucan　has　received　extensive　attention　for　its　physiological　effects　on　lowering　cholesterol　and　postprandial　blood　glucose　levels.　The　benefits　are　generally　ascribed　to　the　viscosity　that　formed　in　gastrointestinal　tract　by　oat　β-glucan.　The　viscosity　depends　on　the　solubility,　molecular　weight,　and　structure　of　oat　β-glucan.　So　these　molecular　characteristics　and　the　effect　of　processing　on　which　are　one　of　the　focus　of　this　review.　The　quantitative　relationship　between　the　hypoglycemic　or　hypocholesterolemic　activities　and　the　physicochemical　characteristics　of　oat　β-glucan　in　food　matrix　will　be　discussed.　Since　cereal　β-glucan　is　an　ingredient　in　functional　food,　many　processes　have　been　developed　to　produce　β-glucan　concentrates　and　isolates,　which　are　widely　applied　in　food　industry.　Like　mushroom-derived　(1→3)-β-glucan,　oat　β-glucan　displays　immune　activity,　the　recent　discoveries　on　immune　function　will　be　given　a　brief　introduction.　©　2011　Nova　Science　Publishers,　Inc.　All　rights　reserved.