A variety of poly(ethylene terephthalate) (PET) fibers having different fine structures were treated with RF discharge low-temperature plasma, and effects on etching were investigated by SEM. In particular, etching profile appeared conspicuously by oxygen plasma on the PET fiber surface was affected by fiber orientation and crystallinity rather than intrinsic viscosity. The investigtion revealed that the PET fiber of low orientation and low crystallinity is randomly etched, that is to say, irrespective of fiber axial direction, whereas the PET fiber of high orientation and high crystallinity is etched at right angles to the fiber axial direction. The regularity intervals are related to the long period of fine structure, suggesting that noncrystalline portion is selectively etched. It has been proved that etching profile of PET undrawn fiber treated by low temperature plasma was changed after streched and this surface topographical structure was very different from that of plasma-treated PET fiber which had been drawn before plasma treatment.