Abstract: Five steel-encased concrete composite beams are tested to study their torsional behaviors, two under pure torsion and three under combined torsion. The torsion-ratio of torsion relationship, steel torsion-strain relationship and strain distribution of steel-encased and concrete are measured. The results show that the torsion carrying capacities of composite beams are undertaken by the concrete flange and the box beam comprising the steel-encased and concrete flange. The bending-torsion ratio is the important factor of the torsion carrying capacity. The ultimate torsion for composite beams will arrive at maximum when the bending-torsion ratio is about 3 and the other conditions are identical. On the basis of elasticity plastic theory, the formula about cracking torsion of pure torsion and combined torsion are presented. By using spatial truss model with variable angle, formulas for ultimate torsion strength of pure torsion and combined torsion are proposed. The predicted results are in good agreement with measured ones.