Reinforced　concrete　(RC)　beam-column　joints　often　present　brittle　shear　failure　in　the　core　area.　The　objective　of　the　study　is　to　study　the　effects　of　stirrup　ratio　and　axial　load　ratio　on　the　shear　capacity　of　geometrically　similar　beam　column　joints　having　different　structural　sizes.　In　this　work,　a　series　of　3D　mesoscale　simulations　on　the　shear　failure　of　RC　beam-column　joints　were　conducted　and　validated,　in　which　the　concrete　heterogeneity　was　considered　explicitly.　The　simulation　results　indicate　that　stirrups　can　improve　the　shear　strength　of　joints,　enhance　the　ductility,　improve　deformation　ability,　while　weaken　the　size　effect　on　the　nominal　shear　strength.　Moreover,　the　increasing　axial　load　ratio　would　lead　to　the　increase　of　shear　strength,　as　well　as　the　increase　of　size　effect.　Based　on　the　modeling　results,　a　novel　semiempirical　and　semitheoretical　size　effect　law　considering　the　quantitative　effect　of　stirrups　and　axial　load　ratio　was　proposed.　In　addition,　the　proposed　size　effect　law　was　verified　against　with　some　available　test　data.