A　steel-concrete　composite　slim　beam,　with　its　steel　section　encased　in　a　concrete　slab,　provides　a　composite　steel-concrete　floor　construction　with　minimum　depth　and　high　fire　resistance.　It　has　become　popular　in　multi-storey　steel　buildings　throughout　Europe　in　recent　years　and　also　been　used　in　some　projects　in　China.　For　the　structural　engineer,　it　is　critical　to　understand　the　behavior　of　composite　slim　beam　in　the　sagging　moment　region.　In　this　paper,　the　bending　capacity　and　flexural　stiffness　of　the　composite　slim　beams　in　sagging　moment　region　are　investigated　theoretically.　Meanwhile,　the　stiffness　and　bending　capacity　are　given　by　Finite　Element　Analysis　(FEA).　The　experiment　of　a　simply　supported　beam　has　been　carried　out　in　order　to　validate　the　theoretical　results.