Using　BFRP　(Basalt　Fiber　Reinforced　Polymer)　bars　instead　of　steel　bars　in　RC　(Reinforced　Concrete)　structures　can　fundamentally　solve　the　problems　caused　by　steel　corrosion.　However,　few　studies　are　currently　being　done　on　the　torsional　performance　of　full-scale　BFRP　bars-RC　members.　To　investigate　the　effect　of　the　stirrups　on　the　torsional　performance　of　BFRP　bars-　RC　beams　with　different　sizes,　12　geometrically　similar　BFRP　bars-　RC　beams　were　designed　with　the　stirrup　ratio　and　specimen　size　as　variables　and　pure　torsion　experiments　were　conducted.　The　test　results　show　that　all　specimens　are　damaged　due　to　diagonal　torsional　cracking,　and　increasing　the　stirrup　ratio　can　improve　the　beams＇　torsional　bearing　capacity,　ductility,　and　deformability.　The　BFRP　bars-RC　beams＇　torsional　strength,　ductility,　and　deformation　capacity　exhibit　a　remarkable　size　effect,　with　the　torsional　strength　decreasing　by　52.32　%　when　the　beam　height　is　increased　from　300　mm　to　900　mm.　Increasing　the　stirrup　ratio　can　weaken　the　beams＇　size　effect,　but　the　size　effect　will　not　be　eliminated.　Furthermore,　the　rationality　of　Jin＇s　size　effect　law　was　verified　through　the　test　values　and　the　collected　pure　torsion　test　database　of　FRP　bars-RC　specimens.　Finally,　the　previously　proposed　calculation　method　for　the　BFRP　bars-RC　beams＇　torsional　bearing　capacity　was　confirmed　to　scientifically　predict　the　torsional　bearing　capacity　of　beams　of　different　sizes.　©　2024　Elsevier　Ltd