Abstract This paper aims to investigate the flexural behavior of concrete filled tubes (CFSTs) made of seamless steel which can handle more pressure than welded steel. Experimental, Theoretical and Finite Element Analyses are utilized for this purpose. The experimental program consists of four-point bending tests of six CFSTs and three hollow steel tubes (STs) for three different Diameter-to-thickness (D/t) ratios of 7.82, 13.5 and 17.5. The test results included are the moment versus displacement and strains, failure modes and ultimate capacities. The contribution of the concrete infill to the flexural capacity was more significant in specimens with higher D/t ratios. All CFST beams exhibited ductile mode of failure with no local buckling. The experimental moments are compared to theoretical nominal moments calculated by well-known international design codes such as the Architectural Institute of Japan (AIJ), the British Standard (BS), the AISC-LRFD, and the Euro code4. Only the AIJ equations predicted non-conservative capacities particularly at the highest D/t ratio. The other codes and standards were more conservative since they did not consider the effect of concrete confinement in their design equations. Finite Element (FE) simulation of the flexural response of CFST is also conducted by developing a nonlinear 3D model considering both material and geometric nonlinearities. The FE model is verified using the present experimental results and a good agreement was achieved in terms of the moment capacity, the failure mode and the moment-mid span deflection curves. In addition, the verified finite element model was used to carry out a parametric study considering wider ranges of D/t ratios and yield strengths.