Double　skin　composite　columns　are　formed　from　two　steel　skins　in　which　filled　with　concrete.　This　paper　describes　a　series　of　tests　of　concrete-filled　double　skin　steel　tubular　(CFDST)　beam-columns.　Both　outer　and　inner　tubes　are　cold-formed　rectangular　hollow　sections　(RHS).　A　theoretical　model　is　developed　in　this　paper　for　the　CFDST　beam-columns.　A　reasonable　good　agreement　is　observed　between　the　predicted　and　tested　curves.　Simplified　models　are　derived　to　predict　the　load-carrying　capacities　of　the　composite　members.