Steel-concrete　composite　structures,　which　possess　the　advantages　of　both　steel　structures　and　concrete　structures,　have　been　quickly　developed　and　widely　used.　Up　to　now,　it　has　been　a　popular　structure　form　for　high-rise　buildings.　Although　composite　structures　have　been　more　and　more　used　in　earthquake　regions,　relevant　fragility　analysis　is　not　available　in　the　literature.　A　performance-based　fragility　analysis　method　is　proposed,　with　uncertainties　due　to　variability　of　ground　motion　characteristics　and　structures　taken　into　consideration.　Four　ultimate　damage　states　for　entire　structure　and　interstory　are　defined,　and　based　on　which,　a　method　for　solving　the　limit　values　of　the　four　performance　levels　is　put　forward.　Seismic　fragility　analysis　is　carried　out　for　two　different　concrete-filled　square　tubular　(CFST)　frames,　and　fragility　curves　are　derived　to　assess　and　compare　the　seismic　performances　of　the　two　structures.