Abstract: At two filament over-feed ratios,different kinds of rotor-spun composite yarns were produced by combining cotton fibers with polyester filaments under varying twisting factors.The yarn tensions were measured and the effects of twist factors on the structure and properties of rotor-spun composite yarns were investigated.Results show that twist factor has great influence on the characteristics of composite yarns at two filament over-feed ratios.The polyester filament in the composite yarn is twisted with the cotton strand and follows a helical path.When the filament tension increases with increasing yarn twist factors and decreasing filament over-feed ratio,the filament yarn tends to lie near the inner layer of the composite yarn and the properties of composite yarns will change accordingly.Compared with the normal rotor-spun yarn,the appearance of composite yarns is clearer,more compacted and the properties are improved.