Most　previous　studies　have　made　great　progress　in　the　size　effect　of　concrete　materials　and　several　classic　size　effect　laws　(SELs)　have　been　proposed.　However,　there　is　a　lack　of　size　effect　laws　for　RC　components　including　beams,　columns,　and　beam-column　joints.　The　focus　of　this　study　is　the　size　effect　in　RC　columns　confined　by　stirrups　under　axial　compression.　A　three-dimensional　mesoscale　numerical　method　for　the　simulation　of　the　failure　of　RC　columns　was　established.　The　numerical　results　were　found　in　good　accordance　with　the　available　experimental　ones,　demonstrating　the　rationality　of　the　simulation　method.　The　simulation　method　was　then　extended　to　model　the　size　effect　of　circular　RC　columns　having　larger　sizes.　The　main　parameter,　stirrups　ratio,　which　evaluates　the　size　and　interaction　effects　between　the　concrete　and　reinforcement,　was　examined　and　analyzed.　Furthermore,　the　influence　mechanisms　of　confinement　effect　generated　by　stirrups　on　the　nominal　compressive　strength　were　explored.　Finally,　based　on　the　available　classic　SEL　for　concrete　materials,　a　predictive　formula　was　built　that　can　describe　the　quantitative　influence　of　constraint　generated　by　stirrups　on　size　effect　of　RC　columns　under　axial　compression.　For　comparison,　the　validation　of　the　predictive　theorical　formula　with　the　available　test　data　is　presented.　(C)　2019　American　Society　of　Civil　Engineers.