Under　the　action　of　earthquake,　reinforced　concrete　(RC)　short　columns　often　undergo　brittle　compression-shear　failure　due　to　the　combined　effects　of　vertical　and　horizontal　loadings.　The　brittle　failure　may　exhibit　obvious　size　effect　of　the　columns.　Considering　the　meso-scale　structural　characteristics　of　concrete,　the　heterogeneity　of　concrete　and　the　nonlinear　interaction　between　steel　bars　and　surrounding　concrete,　a　three-dimensional　meso-scale　numerical　model　is　established　to　study　the　mechanical　behavior　and　seismic　performance　of　reinforced　concrete　short　columns　with　different　structural　sizes.　The　meso-scale　simulation　results　are　in　good　agreement　with　the　available　experimental　results.　Moreover,　the　effect　of　the　axial　compression　on　the　seismic　performance　of　RC　short　columns　with　different　sizes,　especially　large-sized　ones　is　investigated,　including　the　shear　bearing　capacity,　hysteretic　behavior,　ductility　and　energy　dissipation　capacities.　Finally,　based　on　the　classical　Size　Effect　Law　(SEL)　proposed　by　Baant,　a　modified　formula　is　proposed　to　reflect　the　size　effect　on　the　shear　capacity　of　reinforced　concrete　short　columns.　©　2018,　Engineering　Mechanics　Press.　All　right　reserved.