The effects of dyeing on the physical property of polypropylene fibers have been studied. From the results of stress-strain measurements, the dyed fibers generally show a lower initial slope compared to the blank dyed reference fiber. This trend is much clearer in the case of fiber specimens with a higher draw ratio. Dyeing causes a higher fiber shrinkage than the blank dyeing. Therefore, the lower initial slope of the dyed fibers may be due to the higher degree of shrinkage. The data further suggersts that the dye molecules accelerate the relaxation of molecular orientation in the amorphous region of fiber structure during the dyeing process. As for the secondary yield stress and breaking strain, on the other hand, the dyed fiber shows a larger value than the reference respectively, regardless of the draw ratio. The a _c peak of dynamic loss modulus, E ″ for dyed fiber shifts to a higher temperature. This probably means that the number of crystalline defects decrease during the dyeing procedure. Thus the increases of the secondary yield stress and the breaking strain for the dyed fiber can be interpreted as a result of dyeing process inducing more perfect crystalline state. However, considering that those changes, mentioned above, take place with a very small dye take-up in fiber, it seems more reasonable to think that a lower initial slope, a higher secondary yield stress and higher breaking strain for the dyed fibers are not solely due to the existence of dye molecule in fiber structure, but also to the structural changes in the amorphous and crystalline regions which are induced during the dyeing process.