Under　the　action　of　earthquake,　the　reinforced　concrete　(RC)　columns　may　subject　to　torsional　moment,　and　the　existence　of　torsion　will　change　the　failure　mode　of　RC　columns.　Moreover,　the　torsional　fracture　of　RC　columns　often　presents　a　brittle　fracture　pattern,　and　thus　may　have　an　obvious　size　effect.　In　this　work,　a　three-dimensional　meso-scale　simulation　approach　was　utilized　to　study　the　torsional　failure　of　RC　columns.　The　influence　of　structural　size,　longitudinal　reinforcement　ratio,　stirrup　ratio　and　cross-sectional　shape　on　torsional　failure　of　RC　columns　was　investigated.　The　results　show　that:　(1)　the　tested　RC　columns　show　brittle　failure　patterns,　the　nominal　torsional　strength　presents　obvious　size　effect;　(2)　the　longitudinal　reinforcement　presents　little　influence　on　the　size　effect;　(3)　columns　with　square　cross-section　present　stronger　size　effect　than　the　ones　with　circular　shape;　(4)　stirrups　can　improve　the　torsional　strength,　while　they　would　weaken　the　size　effect　on　torsional　strength.　In　addition,　a　novel　size　effect　law　that　can　describe　the　quantitative　influence　of　stirrup　ratio　was　established.　Finally,　based　on　the　variable　angle　truss　model,　the　formulas　for　calculating　the　pure　torsional　capacity　of　RC　columns　were　modified,　considering　the　quantitative　influence　of　the　stirrup　ratio　on　the　size　effect.