After applying torsion to cylindrical 304 stainless steel samples, a gradient structure along the radial direction was obtained. It was found that the volume fraction of α'-martensite increased gradually from the center to the surface of samples. The possibility of deformation-induced martensitic transformation was analyzed theoretically, whose conclusions were consistent with the experimental results. It was found that torsional deformation could produce abundant deformation twin and deformation-induced martensite, which could affect the tensile property of steel distinctly. The existence of deformation and martensite during torsion could increase the strength but deteriorate the ductility dramatically during the tensile test, which should be attributed to the weakening of work hardening ability. As a result of severe deformation, both strain-induced and stress-induced martensite have been observed. Also, two types of martensitic transformation mecha-nisms during torsional deformation were discussed.