This study investigates the concrete filled double skin Tubular (CFDST) beam compared to the concrete filled steel tubular (CFST) beam under pure bending. Normal concrete is used with a compressive strength of 35MPa and an aggregate size ranging from 4.75mm-14mm. Thickness of all low alloy steel tubes are 2.3mm with outer and inner width being 100mm and 50mm, respectively. The results show that the ultimate strength of the CFDST beam was higher by8.2 compared to the corresponding CFST beam. Furthermore, the energy absorption, ductility index, and initial flexural stiffness of CFDST beam were 38.6, 38.3, and 19.02, respectively, higher than the corresponding CFST beams. Moreover, the results indicate that the aggregate size ranging from 4.75-14 mm used in this study is suitable for both CFST and CFSDT elements. © 2022 IEEE.