Compared　with　those　of　concrete　materials,　the　failure　modes　and　mechanisms　of　reinforced　concrete　(RC)　members　are　much　more　complicated.　It　is　therefore　hard　to　describe　the　size　effect　behavior　of　RC　members　by　using　the　size　effect　theory　of　concrete　materials.　From　the　mesoscopic　view,　a　three　dimensional　meso-scale　numerical　analysis　model　of　RC　cantilever　beams　was　established　to　study　the　size　effect　of　shear　failure　of　RC　cantilever　beams.　The　feasibility　of　the　simulation　method　was　verified　by　the　existing　experimental　data,　and　the　influences　of　shear　span　ratio　and　stirrup　ratio　on　the　size　effect　of　shear　failure　of　RC　cantilever　beams　was　investigated.　The　simulation　results　indicate　that:　the　shear　span　ratio　has　a　great　influence　on　the　shear　capacity　of　RC　cantilever　beams,　but　has　little　effect　on　the　size　effect;　the　increase　of　stirrup　ratio　increases　the　shear　capacity　of　RC　cantilever　beams,　and　weakens　the　size　effect　of　the　shear　strength　of　RC　cantilever　beams.　Furthermore,　combined　with　the　influence　mechanism　of　shear　span　ratio　and　stirrup　ratio　on　shear　strength　of　RC　cantilever　beams,　a　theoretical　formula　for　the　size　effect　of　shear　strength　of　RC　cantilever　beams　was　established　based　on　the　theory　of　the　Size　Effect　Law　(SEL)　proposed　by　Baant.　The　accuracy　and　rationality　of　the　theoretical　formula　are　verified　by　comparing　the　simulation　results　and　the　test　data.　©　2020,　Engineering　Mechanics　Press.　All　right　reserved.