Two　kinds　of　CFDSST　members　are　studied　in　this　paper.　The　first　kind　of　them　is　that　with　CHS　(circular　hollow　section)　inner　and　SHS　(square　hollow　section)　outer,　and　the　second　kind　of　them　is　that　with　both　outer　and　inner　tubes　are　CHS　sections.　The　influence　of　parameters,　such　as　the　axial　load　ratio,　hollow　section　ratio,　slenderness　ratio　and　eccentricity　is　investigated.　Theoretical　models　are　presented　to　predict　the　load-carrying　capacities　of　the　composite　members,　and　the　predicted　results　are　in　good　agreement　with　those　of　tests.