The effect of a reactive compatibilizer on the morphology, and thermal and rheological properties of alloy fibers obtained from poly(ethylene terephthalate) (PET)/polyamide 6 (PA6) blends was investigated. An epoxy of bisphenol-A type was selected as the reactive compatibilizer for preparing the polymer alloy fibers by enhancing the compatibility of immiscible blends of PA6 and PET. The PET/PA6/epoxy alloy is expected to be used for preparing practically useful alloy fibers with various advantages such as low cost and high moisture absorption. The PA6/PET/epoxy alloy was successfully prepared via melt compounding by using a twin-screw extruder. The domain size of the poor component decreased enough to result in alloy fibers with reasonable mechanical properties. Further, it was found that the cold crystallization temperature of PA6 in the alloy decreased with increasing epoxy amount, but that of PET increased owing to the significant difference between the crystallization rates of PET and PA6. The $T_g$ values for both polymers in the alloy showed a shift along the direction between them. All the results implied that chemical reactions occurred between epoxy and end groups of PA6 and PET, resulting in improved compatibilization.