In this study, the effects of poly(hedral oligomeric silsesquoxane) (POSS) type and loading level on the mechanical, morphological and thermal properties of melt-spun PET-based composite fibers were investigated. Three different epoxy, hydroxyl or amine functional POSS types were compared. The thermal, morphological and mechanical properties were investigated. It was found that all the three POSS types generally demonstrated nucleating agent behavior, suggested by lower cold crystallization temperatures along with higher melt crystallization temperatures. Among the three types of PUSS particles, addition of 1 wt% hydroxyl functional POSS resulted in five times increased tensile modulus in comparison to the neat PET fiber. This can be explained by the combined effect of reinforcement with higher level of crystalline content in addition to the presence of more oriented amorphous domains. On the other hand, epoxy terminated POSS/PET composite fibers exhibited doubled tensile modulus with respect to the neat PET fiber; however, the results obtained on amine terminated APOSS-PET fiber did not have any significant trend. (C) 2013 Elsevier Ltd. All rights reserved.