This　paper　develops　a　mechanics　model　that　can　predict　the　behaviour　of　concrete-filled　hollow　structural　section　(HSS)　beams.　A　form　of　unified　theory,　where　a　confinement　factor　()　was　introduced　[Steel　Compos.　Struct.-Int.　J.　(2001)　1(1)　51]　to　describe　the　composite　action　between　the　steel　tube　and　filled　concrete,　is　used　in　the　analysis.　A　series　of　concrete-filled　square　and　rectangular　tube　beam　tests　were　carried　out.　The　main　parameters　varied　in　the　tests　were　the　depth-to-width　ratio　(beta)　from　1　to　2,　and　tube　depth　to　wall　thickness　ratio　from　20　to　50.　The　load　vs.　lateral　deflection　relationship　was　established　for　concrete-filled　HSS　beams　both　experimentally　and　theoretically.　The　predicted　curves　of　load　vs.　mid-span　deflection　are　in　good　agreement　with　the　presented　test　results.　Formulas　which　should　be　suitable　for　incorporation　into　building　codes　are　developed　for　calculating　the　moment　capacity　of　concrete-filled　HSS　beams.　Comparisons　are　made　with　predicted　beam　capacities　and　flexural　stiffness　using　the　existing　codes,　such　as　AIJ-1997,　BS5400-1979,　EC4-1994,　and　LRFD-AISC-1999.　(C)　2003　Elsevier　Ltd.　All　rights　reserved.