This　paper　presents　experimental　and　numerical　investigations　on　circular　concrete-filled　double　steel　tubular　(CFDST)　members　under　flexural　load.　Seven　specimens　including　five　CFDST　specimens　were　tested　and　the　test　parameters　included　the　effects　of　concrete　compressive　strength,　the　thickness　of　the　steel　tubes,　and　the　diameter　ratio　on　their　flexural　performance.　The　final　failure　modes,　moment　(M)-curvature　(phi)　curves,　deflection　curves　of　specimens,　and　distributions　of　section　longitudinal　strains　were　analyzed　and　reported　in　this　study.　A　nonlinear　finite　element　model　(FEM)　was　then　developed　for　the　CFDST　members.　After　successful　validation　in　ultimate　strengths　and　failure　modes,　parametric　studies　were　performed　by　using　the　verified　FEM.　A　simple　formula　for　calculating　the　flexural　capacity　of　CFDST　members　was　proposed　and　compared　against　several　design　codes　to　investigate　the　design　accuracy.　Based　on　this　study,　it　was　found　that　the　CFDST　members　exhibit　ductile　behavior　under　flexural　load.　Increasing　the　steel　area,　particularly　for　the　outer　tube　improved　the　flexural　capacities　of　the　members.　Furthermore,　the　existing　design　codes　were　found　to　be　remarkably　underestimating　the　moment　capacities　of　CFDST　members　whereas　the　design　formula　proposed　can　predict　the　ultimate　flexural　capacity　with　reasonable　accuracy.