Axial　compression　tests　of　circular　concrete-filled　steel　tubes　with　different　diameters　(219　mm,　426　mm,　and　630　mm)　and　ratios　of　tube　diameter　to　steel　thickness　(55　and　88)　were　conducted　to　investigate　the　effect　of　size　on　the　bearing　capacity.　The　experimental　results　indicated　that　the　peak　nominal　stress　decreased　as　the　size　increased,　and　the　decrease　in　the　nominal　stress　due　to　the　size　effect　increased　at　higher　ratios　of　diameter　to　thickness.　At　the　peak　load　moment,　an　increased　specimen　diameter　corresponded　to　a　decreased　hoop　stress　in　the　steel　tube　as　well　as　a　decreased　concrete　strength　due　to　the　confinement　effect　of　the　steel　tube.　When　the　ratio　of　diameter　to　thickness　increased,　the　extent　of　reduction　of　the　hoop　stress　and　the　confining　effect　of　the　steel　tube　influenced　by　the　increasing　specimen　size　increased.　However,　the　vertical　stress　in　the　steel　tube　was　increased　at　increased　size,　and　increases　in　the　ratio　of　diameter　to　thickness　improved　the　increase　degree　of　the　vertical　stress　of　steel　tube　due　to　the　enlargement　of　specimen　size.　Hence,　the　vertical　bearing　capacity　of　the　steel　tube　was　affected　by　both　the　specimen　size　and　the　ratio　of　diameter　to　thickness.　Based　on　the　size　effect　law　(SEL)　proposed　by　Bazant,　and　taken　the　effect　of　the　ratio　of　diameter　to　thickness　into　consideration,　a　size-dependent　formula　to　evaluate　hoop　stress　in　the　steel　tube　was　developed.　A　size-related　model　considering　situations　with　different　ratios　of　diameter　to　thickness　was　established　in　order　to　estimate　the　bearing　capacity　of　large-size　circular　concrete-filled　steel　tubes.　The　model　and　experimental　results　showed　good　agreement.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.