Abstract: A study was conducted to investigate the composite action in steel2concrete composite beams subjected to torsion. Tests and analysis showed that the composite action definitely existed under torsion. It would underestimate the torsional strength of composite beams if the interaction was neglected between steel beams and concrete flanges. Steel beams play a vital role in restraining concrete flanges from deforming longitudinally. A three2dimensional behavioral truss model capable of analyzing composite beams sections subjected to combined bending and torsion is presented. It is based on the rotating2angle softened truss model (RA2STM) and assumes that plane sections remain plane to account for composite actions. In this model, the section is assumably subjected to one2 and two2dimensional stresses separately. The former is resisting the longitudinal stresses due to bending and torsion, while the latter is resisting the shear stresses due to torsion. Links between the two systems are compatibility of strains and equilibrium of stresses in the longitudinal direction. It satisfies the equilibrium equations, compatibility equations and the constitutive laws of materials. Through analyzing several specimens available in the literature the results predicted by this method are in good agreement with those obtained from tests. In addition, a reported formula is appraised.