Porous poly(ethylene terephthalate) (PET) fibers were fabricated by the meltspinning method. Here, reactive polymer-functionalized aligned multiwalled carbon nanotube bundles (RP@ACNTB) were introduced into a spinning system to create nanopores in PET fibers via the diffusion of gaseous products evolved from the decomposition of polymers. PET fibers with RP@ACNTB had outstanding mechanical property due to the CNT alignment and the good compatibility between PET and RP@ACNTB. As compared to PET fibers, PET fibers with 0.3-0.8 wt % RP@ACNTB displayed significantly increased tensile strength and improved thermal stability. Especially, the formation of a porous structure could markedly increase the water absorption of PET/RP@ACNTB fibers from 0.9% to 2.9-5.7%. When the same amount of fillers was applied, PET fibers with RP@ACNTB showed better water absorption and lower mechanical property than PET fibers with pristine ACNTB mainly because of the presence of pores in the PET matrix.