The change of elongation property in the melt spinning process of polyethylene terephthalate (PET) fibers, mixed with small amount of additive copolymer less than 5% by weight, was studied. The additive polymer was synthesized to improve the extensibility of matrix PET in the spinning process. The amount, molecular weight of additive polymer, and spinning conditions were changed to investigate the extensibility of as-spun fibers. Experimental results show that the blend of copolymer improves the extensibility of as-spun PET fibers. The elongation at break of as-spun fibers increases with molecular weight and amount of additive polymer. The additive polymer prevents the fiber orientation and this causes the increase of extensibility of as-spun fibers. (c) 2006 Wiley Periodicals, Inc.