Lightweight　aggregate　concrete　(LWAC)　is　more　and　more　utilized　in　engineering　structures　because　of　the　advantages　of　light　weight　and　good　thermal　insulation　performance.　Both　lightweight　aggregate　concrete　and　ordinary　concrete　were　regarded　as　three-phase　composites　consisting　of　the　aggregate　particles,　mortar　matrix　and　interface　transition　zones　(ITZs)　between　them.　A　3D　meso-scale　mechanical　model　was　established　to　simulate　the　shear　failure　behavior　of　concrete　beams　without　web　reinforcement.　The　shear　failure　modes　and　failure　mechanism　of　ordinary　and　light　aggregate　concrete　cantilever　beams　with　different　structural　sizes　under　monotonic　loading　were　modelled　and　studied.　The　size　effect　on　the　nominal　shear　strength　was　examined.　In　addition,　the　accuracy　and　safety　of　the　formula　for　calculating　the　shear　capacity　of　the　relevant　design　codes　are　preliminarily　discussed　based　on　the　simulation　results.　The　results　show　that　different　from　the　ordinary　concrete　beam,　the　LWAC　specimens　sustained　serious　damage　due　to　the　lower　strength　of　the　aggregates.　The　shear　failure　modes　of　concrete　beams　with　different　sizes　were　basically　the　same.　An　obvious　size　effect　on　the　nominal　shear　strength　of　both　LWAC　and　ordinary　concrete　specimens　was　observed.　Compared　with　the　ordinary　concrete　beams,　lightweight　aggregate　concrete　beams　showed　a　stronger　size　effect　in　shear　failure　behavior.　©　2020,　Engineering　Mechanics　Press.　All　right　reserved.