A plastic deformation whose strain trajectory has constant curvature is investigated on mild steel by applying the combined load of axial force and torsion to thin-walled tubular specimen. The experiment is performed by using a full-automatic complex loading testing machine. Experimental result shows that the relation between the magnitude of stress vector and the arc length of plastic strain trajectory depends mainly on the curveture, and the curve showing such a relation appears higher than the hardening curve for pure torsion. On the other hand, cyclic loading makes the curve lower than the hardening curve for pure torsion owing to the effect of cyclic softening. This may be an experimental evidence that the conventional flow theory is not satisfactory for complex loading. However, the flow theory may be found as a sufficient approximation to plastic deformation under complex loading with strain trajectory of constant radius of curvature larger than 2 percent.