Very　few　studies　have　been　dedicated　to　the　size　effect　on　the　shear　behaviour　of　reinforced　concrete　(RC)　columns.　In　particular,　there　lacks　a　design　equation　that　can　quantitatively　represent　the　effect　of　related　parameters　on　the　size　effect　on　nominal　shear　strength　for　RC　columns.　This　work　presents　a　mesoscale　numerical　analysis　of　the　mechanical　response　of　geometrically　similar　RC　columns　subjected　to　constant　axial　load　and　monotonic　lateral　load.　In　the　numerical　model,　the　inherent　heterogeneous　nature　of　concrete　and　steel/concrete　interaction　were　considered.　The　numerical　model　was　validated　against　test　results　in　the　literature,　and　then　adopted　to　conduct　a　parametric　study　of　the　shear　behaviour　and　size　effect　of　RC　columns.　The　investigated　parameters　involved　the　shear-span　ratio,　stirrup　ratio　and　axial　load　ratio.　The　results　indicated　that　the　nominal　shear　strength　presents　an　obvious　size　effect.　This　size　effect　on　nominal　shear　strength　was　gradually　weakened　as　the　stirrup　ratio　and　shear-span　ratio　increased.　Moreover,　considering　the　quantitative　influence　of　stirrup　ratio　and　shear-span　ratio　on　size　effect,　a　modified　equation　was　derived　to　predict　the　shear　resistance　of　RC　columns.　Through　comparing　with　available　experimental　results,　the　modified　equation　was　demonstrated　to　be　accurate　and　reliable.