In　recent　years,　it　was　proposed　by　several　researchers　that　concrete　filled　double　skin　steel　tubes　(CFDST)　be　studied　for　their　strength　as　a　column　or　a　beam.　Advantages　of　CFDST　over　fully　concrete　filled　steel　tubes　(CFST)　include:　increase　in　section　modulus;　enhancement　in　stability;　lighter　weight;　better　damping　characteristics　and　better　cyclic　performance.　It　is　thus　expected　that　concrete　filled　double　skin　steel　tubes　(CFDST)　have　the　potential　of　being　used　in　building　structures.　This　paper　provides　new　test　data　pertaining　to　the　seismic　behavior　of　CFDST　beam-columns.　The　test　parameters　included　the　section　types　(circular　and　square),　the　core　concrete　strength　(f(cu)),　and　the　axial　load　level　(n).　Twenty-eight　CFDST　column　specimens,　including　16　specimens　with　SHS　(square　hollow　section)　outer　and　CHS　(circular　hollow　section)　inner,　12　specimens　with　CHS　(circular　hollow　section)　outer　and　CHS　(circular　hollow　section)　inner　were　tested　under　constant　axial　load　and　cyclically　increasing　flexural　loading.　The　CFDST　beam-columns　were　found　to　have　significant　increase　in　strength,　ductility,　and　dissipated　energy　over　the　outer　jackets.　In　general,　the　ductility　and　energy　dissipation　ability　of　specimens　with　circular　sections　are　higher　than　those　of　the　specimens　with　square　sections.　The　mechanics　model　which　was　developed　by　the　authors　for　concrete　filled　steel　tubular　(CFST)　beam-columns　subjected　to　constant　axial　load　and　cyclically　increasing　flexural　loading,　is　used　to　analyze　the　behavior　of　CFDST　beam-columns.　It　is　found　that　the　predicted　cyclic　responses　for　the　composite　beam-columns　are　generally　in　reasonable　agreement　with　test　results.　Finally;　comparisons　are　made　with　predicted　beam-column　flexural　stiffness　using　the　existing　codes.　(c)　2006　Elsevier　Ltd.　All　rights　reserved.