The　sources　of　size　effect　on　RC　members　are　the　heterogeneity　of　concrete　material　and　the　interaction　between　steel　and　concrete.　From　the　view　of　microcosmic,　a　three-dimensional　micromechanical　model　for　reinforced　concrete　beams　was　set　up.　In　the　model,　the　concrete　heterogeneity　was　considered,　and　the　interactions　between　steel　and　concrete　was　described　by　a　nonlinear　spring　element.　Base　on　the　well　agreement　between　simulation　results　and　experimental　results,　the　failure　behavior　of　large　beams　was　studied,　meanwhile,　the　change　of　failure　pattern　and　flexural　strength　of　different　size　beams　under　monotonic　and　cyclic　loading　were　analyzed.　The　simulation　results　show　that:　1)　in　the　working　condition　of　this　article,　the　flexural　of　RC　cantilever　beams　have　the　size　effect,　and　the　flexural　strength　decreases　with　the　increase　of　depth　of　the　beam;　2)　under　cyclic　loading,　the　bending　beam　is　brittle　because　of　the　fatigue　of　the　bonding　performance　between　concrete　and　steel;　3)　compared　to　monotonic　loading,　the　cantilever　beam　damage　is　more　brittle　and　the　size　effect　of　nominal　flexural　strength　is　more　obvious　under　a　cyclic　loading　condition.　©　2018,　Engineering　Mechanics　Press.　All　right　reserved.