Abstract: Mechanical analysis of the geometric disposition of single yarns in a ply yarn,the ply yarn structure,and the effect of twisting on increasing the strength of the ply yarn was carried out under the conditions that the ply yarn was bearing tensional load.The conditions for forming a solid core ply yarn are pointed out,and conclusions are drawn that the strengthening effect of twisting on the hollow core ply yarn is greater than that on the solid core ply yarn.Twisting together in one operation,single yarns or strands should not be more than five.When a high strength ply yarn is desired,multi-doubling-twisting processes are proposed.