1）The performance of fiber The higher the fiber strength, the lower the linear density. And the longer the length, the higher the yarn tenacity. If the chemical crimp is small, which will increase the fiber adhesion force, thus improving the yarn strength. 2）Yarn structure The yarn strength can be improved when the fiber is transferred in the direction of yarn radius and the structure is uniform. Yarn strength is reduced when the fibers are arranged in layers, or when the fibers are folded, flexed, or hooked. Yarn strength increases with the increase of twist below the critical twist, and decreases with the increase of twist after the critical twist is exceeded. Pure cotton yarn is easy to break in details and large knots. 3）Atmospheric temperature and humidity Yarn strength decreases as temperature increases. With the increase of relative humidity, the yarn moisture recovery rate increases, and the yarn strength of cotton and flax increases, while the yarn strength of wool and artificial fiber decreases. Therefore, the strength test should be carried out after equilibrium at the specified standard state (temperature 20 ℃, relative temperature 65%), otherwise the measured strength should be corrected for temperature and moisture regain.