With　the　devolvement　of　large-span　structures　and　super　high-rise　buildings,　lightweight　aggregate　concrete　(LWAC)　materials　have　been　widely　utilized.　The　main　content　of　this　work　is　to　explore　the　failure　and　size　effect　of　lightweight　aggregate　concrete-filled　steel　tube　(LWACFST)　columns　under　uniaxial　compression.　A　three-dimesnional　mesoscale　simulation　approach　considering　concrete　heterogeneity　was　developed,　in　which　the　ideal　bond　was　assumed　between　the　steel　tube　and　inner　concretes.　The　failure　of　geometrically　similar　square　and　circular　LWACFST/CFST　columns　under　axial　compression　having　different　structural　sizes　and　transverse　constraints　was　modeled　and　explored.　The　size　effect　on　the　nominal　compressive　strength　of　the　columns　was　examined.　The　influences　of　aggregate　type,　column　cross-sectional　type,　and　constraint　effect　generated　by　the　steel　tube　on　the　size　effect　in　axial　compressive　failure　are　also　investigated.　The　results　indicate　that　an　obvious　size　effect　in　axial-compressive　failure　can　be　found　for　both　LWACFST　and　CFST　columns.　With　the　increase　of　the　confinement　degree　of　steel　tube,　the　size　effect　on　nominal　compressive　strength　would　be　weakened.　The　LWACFST　columns　exhibits　a　more　obvious　size　effect　than　the　CFST　columns.　Moreover,　the　size　effect　on　the　compressive　strength　of　the　circular　columns　is　obviously　weaker　than　the　effect　on　square　columns.　Finally,　considering　the　quantitative　influence　of　confinement　coefficient,　a　novel　size　effect　law　(SEL)　for　quantitatively　describing　the　size　effect　in　axial-loaded　CFST　columns　was　proposed.　The　accuracy　and　reasonability　of　the　proposed　SEL　was　verified　by　the　available　test　data.　DOI:　10.1061/(ASCE)ST.1943-541X.0002827.　(c)　2020　American　Society　of　Civil　Engineers.