The　objectives　of　this　study　are,　on　one　hand,　to　present　an　experimental　campaign　on　the　flexural　failure　behavior　of　RC　beams　under　both　low　cyclic　and　monotonic　lateral　loadings,　and　on　the　other　hand,　to　discover　the　effect　of　cross-section　size　on　the　global　mechanical　properties　of　RC　beams.　Five　groups　of　geometrically　similar　RC　cantilever　beams,　with　cross-section　sizes　range　from　200　mm　to　1000　mm　and　a　shear-span　ratio　of　4　(length　to　shear　span)　were　experimentally　tested.　Moreover,　a　3D　meso-scale　numerical　method,　which　could　consider　both　the　heterogeneity　of　concrete　material　and　the　nonlinear　interaction　between　reinforcing　bars　and　surrounding　concretes,　for　the　simulation　of　failure　of　RC　members　was　developed,　and　some　numerical　tests　were　established　based　on　the　experimental　campaign.　The　tested　results　demonstrate　the　presence　of　size　effect　in　flexural　failure　of　RC　beams　under　both　low　cyclic　and　monotonic　lateral　loadings.　Specifically,　the　effects　of　structural　size　on　the　seismic　performances　of　the　RC　beams,　involving　the　failure　pattern,　the　ductility,　the　stiffness　degradation　and　the　load　carrying　capacity,　were　extensively　investigated　based　on　the　combined　experimental　and　numerical　tests.　Under　cyclic　loading,　due　to　the　low　cyclic　fatigue　damage　behavior　and　for　the　fact　that　failure　always　takes　place　in　concrete　which　is　a　quasi-brittle　engineering　material,　all　the　tested　beams　could　exhibit　a　more　quasi-brittle　failure　pattern,　and　consequently　the　flexural　failure　of　the　beams　presents　a　stronger　size　effect　compared　with　that　under　monotonic　loading.