In this study, it was aimed to compare the mechanical, thermal and morphological properties and dimensional stability of organoclay or poly(hedral oligomeric silsesquioxane) (POSS) reinforced poly(ethylene terephthalate) (PET) fibers to be potentially used in tire manufacturing as the cord. Clay and POSS have different particle structure as a reinforcing agent. Therefore, they tend to disperse and interact differently in polymer matrix. Three different phosponium salts were used to synthesize organoclays. Among them tributylhexadecyl phosphoniumbromide (TBHDPB) exhibited the best properties, hence it was decided to use in fiber preparation. On the other hand, three different POSS types including epoxy, amine or trisilanol functional groups, expected to interact with PET, were utilized. PET/Clay and PET/POSS nanocomposite fibers were prepared by melt compounding and subsequent monofilament fiber spinning processes in Xplore Instrument Fiber Line (Holland). Mechanical tests showed an increased elastic modulus and strength at break by the addition of organoclay or POSS particles. It was observed that 10-factor fiber drawing was the major step affecting the mechanical properties. Drawing also improved the %-crystallinity of neat and nanocomposite fibers. The influence of nanoparticle type on the mechanical performance of PET fibers was compared.