The mathematical model of peripheral twist extent (PTE) length was set up by studying the kinematics and dynamics of the yarn in rotor groove. The value of PTE length was calculated by using numerical method, and the factors affecting the PTE length were also discussed. The results show that the theoretical predicted value of PTE length seems to be consistent with the measured value. The yarn linear density and rotor speed have remarkable effects on PTE length. The parameters including twist multiplier, navel radius and doffing tube radius, rotor diameter, and the rotor groove angle are certain to influence on PTE length.