Thermoplastic poly(ethylene terephthalate) (PET) fibers have been used to toughen an intrinsically brittle epoxy resin with high glass transition temperature. The morphologies and chemical properties of the as-received and the surface-modified PET fibers are comparatively investigated by scanning electron microscopy (SEM) and time-of-flight static secondary ion mass spectrometry (ToF-SIMS). Strong fiber-matrix adhesion is successfully achieved by optimizing surface modification of PET fibers with NaOH. Compared with neat epoxy resin, the fracture toughness of PET fibers filled epoxy composites is almost doubled when loading only 1 wt% chemically treated fiber. The fracture behavior and toughening mechanisms are studied by SEM observations on the fracture surfaces. Good correlation is found between the surface morphologies of the treated fibers, the fracture surfaces and the observed fracture toughness. SEM shows that fiber pullout, fiber breakage and fort-nation of step structures due to fiber addition are among the operative toughening mechanisms observed.