This paper deals with the tensile behaviour of rotor spun yarns. Having regard to the special construction of rotor spun yarn, some modifications for Hearle's theory have been made morder to apply Hearle's theory to rotor spun yarn. The paper then goes into calculating the tensile curve and investigating the effect of main parameters and spinning conditions on the tensile behaviour of rotor spun yarn. It was found that the following facts are noteworthy for improving the mechanical properties of rotor spun yarn: (l) Selecting suitable navel so as to control the P. T. E. length within a reasonable range. (2) Improving the design of single fibre supply device to decrease the amount of spoiled fibre and hence increase the effective fibre length. (3) Increasing spinning yarn tension without yarn break to enhance the fibre migration ratio. (4) For the rotor spun yarn of industrial use, it is the way to increase the yarn strength that the rotor groove should be designed with the groove angle beneficial to decrease the yarn specific volume.