A　3D　meso-scale　simulation　model　was　developed　to　study　the　failure　behavior　of　reinforced　concrete　(RC)　beams,　in　which　the　concrete　heterogeneities　and　the　nonlinear　concrete-steel　interaction　were　taken　into　account.　The　developed　meso-scale　simulation　method　was　then　validated　by　comparing　the　simulation　results　with　the　available　tests　for　RC　cantilever　beams.　In　addition,　the　effects　of　stirrups　ratio,　shear-span　ratio　and　loading　mode　on　the　size　effect　in　shear　strength　of　RC　beams　with　a　maximum　depth　of　2　000　mm　were　explored.　The　simulation　results　indicate　that:　1)　The　shear　strength　of　beams　without　web　reinforcement　presents　obvious　size　effect,　and　the　influence　coefficient　of　section　height　proposed　by　Chinese　Code　for　design　of　concrete　structures　cannot　fully　reflect　the　size　effect　of　RC　beams;　2)　The　web　reinforcements　can　inhibit　size　effect　on　the　shear　strength　of　RC　beams,　and　the　size　effect　on　shear　strength　of　beams　gradually　weakens　with　the　increase　of　stirrups　ratio;　3)　As　the　shear-span　ratio　increases,　the　shear　capacity　of　the　beam　decreases,　and　the　size　effect　becomes　to　be　weakened;　4)　Compared　with　monotonic　loading,　cyclic　loading　leads　to　the　low-cyclic　fatigue　failure　of　RC　beams,　and　thus　it　makes　the　size　effect　in　shear　strength　more　prominent.　©　2019,　Editorial　Department　of　JCEE.　All　right　reserved.