The　main　purpose　of　this　study　was　to　investigate　the　effect　of　the　spacing　of　through　bolts　on　the　performance　of　the　steel-aluminum　composite　mullions　(SACM)　using　finite　element　analysis,　and　the　equations　for　the　stress　and　composite　stiffness　at　the　midspan　of　the　SACM　considering　the　coefficient　of　shear　resistance　eta　were　proposed.　The　results　shown　that　the　ultimate　bearing　capacity　of　the　SACM　with　a　full　composite　action　is　increased　by　18%　and　65%　as　compared　with　that　of　the　SACM　with　no　composite　action　and　the　pure　aluminum　alloy　mullion.　The　deflection　at　the　midspan　of　the　SACM　decreases　with　the　bolt　spacing　decreases,　indicating　that　the　stiffness　of　SACM　increase　with　the　bolt　spacing　decreases,　because　SACM　can　enhance　the　material　utilization　of　aluminum　alloy　and　the　capability　of　cooperative　work　of　steel　and　aluminum　alloy,　in　addition,　the　bolt　spacing　that　equals　a　third　of　the　span　is　sufficient　to　allow　the　steel　and　aluminum　to　have　a　desired　composite　action.　The　equations　for　the　stress　and　composite　stiffness　at　the　midspan　of　the　SACM　under　the　condition　of　partial　shear　connection　are　derived,　and　the　results　calculated　from　these　equations　agree　well　with　that　obtained　by　the　finite　element　method.