The glass transition behavior and morphology of the acrylic acid-grafted PET fiber were studied. The glass transition behavior was analyzed by using differential scanning crlorimetry: the morphology, by optical and electron microscopy. As the amount of grafting increased, the glass transition temperature (Tg) seemed to slightly move toward a low temperature region whereas the melting temperature showed no appreciable shift. The grafted PET showed improved dyeability for C.I. Basic Red 95 and increased moisture regain. The overall morphological results indicated that grafting has occurred in both surface and inner side of fiber.