To　better　understand　the　differences　in　torsional　performance　and　size　effect　between　BFRP-reinforced　concrete　(BFRP-RC)　beams　and　reinforcement　concrete　(RC)　beams,　numerical　tests　on　the　pure　torsional　performance　of　BFRP-RC　and　RC　beams　was　carried　out　by　a　3D　meso-scale　numerical　method.　The　effects　of　beam　height,　stirrup　ratio,　and　reinforcement　type　on　torsional　damage　and　the　size　effect　law　of　beams　were　analyzed.　Furthermore,　the　simulation　results　were　compared　with　national　codes　and　Jin　et　al.＇s　size　effect　law.　The　results　indicate　that:　1)　With　the　beam　size　increase,　the　torsional　capacity　and　ductility　of　BFRP-RC　beams　decrease　more　than　that　of　RC　deep　beams.　2)　Stirrups　can　not　only　improve　beams＇　torsional　strength　but　also　weaken　the　torsional　size　effect,　but　the　effect　of　BFRP　stirrups　is　not　as　evident　as　that　of　steel　stirrups.　3)　The　safety　reserve　of　calculation　results　in　national　codes　is　insufficient　for　the　torsional　capacity　of　large-size　beams.　Moreover,　the　contribution　of　concrete　should　be　reasonably　considered　in　the　pure　torsional　capacity　of　concrete　members.　4)　The　size　effect　law　proposed　by　Jin　et　al.＇s　can　well　describe　the　torsional　size　effect　of　BFRP-RC　beams　and　RC　beams.　©　2023　Elsevier　Ltd