The　failure　modes　and　mechanisms　of　reinforced　concrete　(RC)　members　are　far　different　from　those　of　the　concrete　material.　It　is　therefore　debatable　to　use　the　size　effect　theory　and　formula　based　on　the　concrete　material　to　describe　the　size　effect　of　RC　members.　Moreover,　the　size　effect　formula　of　the　concrete　material　is　difficult　to　describe　the　influence　of　other　important　structural　parameters　on　the　size　effect　of　RC　structural　members.　Taking　circular　concrete　columns　confined　by　stirrups　for　the　research　objects,　combined　with　the　physical　test　and　numerical　results,　the　main　parameter　of　stirrup　ratio　that　affects　the　size　effect　of　nominal　compressive　strength　was　revealed.　Furthermore,　combined　with　the　influence　mechanism　of　the　confining　effect　on　the　axial　compressive　strength　of　RC　columns　and　based　on　the　basic　theory　of　the　Size　Effect　Law　(SEL)　proposed　by　Baant,　a　semi-theoretical　and　semi-empirical　formula　for　the　size　effect　of　the　uniaxial　compressive　strength　of　stirrup-confined　RC　columns　was　established.　The　experimental　and　numerical　results　in　the　present　study　as　well　as　other　experimental　data　verify　the　accuracy　and　rationality　of　thesemi-theoretical　and　semi-　empirical　formula　of　the　size　effect　on　the　RC　member　level.　©　2020,　Engineering　Mechanics　Press.　All　right　reserved.