Abstract: This paper described the state of breaking elongation of the ring-spun yarn and analyzed the role of twist in the ring-spun yarn and the force acting on fibers of the yarn under tensile load.It is considered that the resolution and transformation of this force as a result of existence of twist is an important contributing factor which enhances the bearing capacity for tensile load of the yarn,thus enheancing its tensile strength.A new concept of critical twist is proposed: the tensile component force at the fiber in yarn axis under tensile load is equal to the tensile strength of the fiber,simultaneouly the shearing component force on the surface layer fiber is equal to the shearing strength of the fiber.