Size　effect　of　reinforced　concrete　(RC)　members　is　mainly　caused　by　the　heterogeneity　and　mutual　effect　of　concrete　and　steel.　Considering　of　the　above　influential　factors,　the　3D　meso-mechanical　models　of　stirrup-confined　concrete　square　columns　were　set　up　and　the　damage　process　subjected　to　axial　compressive　loading　was　simulated.　The　confinement　effect　on　the　failure　patterns　and　the　axial　compressive　strength　of　the　stirrup-confined　columns　was　explored.　Moreover,　the　simulated　results　were　compared　with　test　data.　Based　on　the　basic　theory　of　the　size　effect　law　(SEL)　proposed　by　Baant,　size　effect　of　stirrup-confined　concrete　columns　was　analyzed.　It　was　showed　that　the　simulated　results　have　a　good　consistence　with　the　test　data　and　the　mesoscopic　analysis　models　of　RC　columns　can　accurately　describe　the　damage　behavior　and　size　effect　of　stirrup-confined　columns.　With　the　increase　of　stirrup　ratio,　the　nominal　strength　and　deformation　behavior　of　columns　boost.　Then,　it　becomes　little　brittle　in　the　damage　process.　Therefor　the　size　effect　is　little　obvious.　The　simulation　data　fit　well　with　basic　theory　of　the　size　effect　law　(SEL)　proposed　by　Baant.　stirrup-confined;　©　2017,　Science　Press.　All　right　reserved.