In　this　paper,　a　3D　meso-scale　numerical　model　for　pure-torsional　failure　analysis　of　carbon　fiber　reinforced　plastic　(CFRP)-strengthened　reinforced　concrete　(RC)　beams　is　established.　Three　main　aspects　regarding　the　establishment　of　the　numerical　model　are　thoroughly　considered,　including　the　heterogeneity　of　concrete,　the　bond-slip　relationship　between　steel　bars　and　concrete,　and　the　nonlinear　interaction　between　CFRP　and　concrete.　Effects　of　CFRP　fiber　ratio　and　span-height　ratio　on　pure-torsional　performances　and　size　effect　behaviors　of　CFRP-strengthened　RC　beams　are　analyzed.　With　the　increase　of　CFRP　fiber　ratio,　the　cracks　spacing　decreases,　the　number　of　cracks　increases,　and　the　strain　of　CFRP　decreases.　With　the　increase　of　the　span-height　ratio,　the　inclination　angle　of　the　oblique　crack　decreases,　and　the　strain　of　CFRP　increases.　It　is　found　that　CFRP-strengthened　RC　beams　have　an　obvious　torsional　size　effect.　Furthermore,　CFRP　fiber　ratio　and　span-height　ratio　not　only　have　a　coupling　enhancement　effect　on　the　nominal　torsional　strength,　but　also　have　a　coupling　weakening　effect　on　the　size　effect　behavior.　Based　on　the　simulation　results,　a　CFRP-related　Torsional　size　effect　law　(SEL),　which　can　quantitatively　describe　the　influences　of　CFRP　fiber　ratio　and　span-height　ratio　on　the　size　effect　of　nominal　torsional　strength　of　CFRP-strengthened　RC　beams,　is　established.　©　2023　Institution　of　Structural　Engineers