Abstract: In this paper, we first have developed a testing machine which can apply the dynamic combined stresses of torsion and tension to specimens, and briefed the experimental method using the equipment. Second, we carried out the dynamic experiments on the mild steel specimens. The experimental results showed that the curves between the equivalent stresses and the equivalent strains were almost paralled when the equiyalent strain rates ranged from 35/s to 56/s under the dynamic combined stress states. The dynamic yield surfaces of the mild steel are coincided with the Von Mises' yield condition very well. The results also showed that the overstress model is true. Finally, we found that the equivalent stress-strain relation is in a single character under the constant equivalent strain rates.