In order to improve hydrophilicity of PET fabric, PET fabric was aminolyzed with 5%, 15%, 25%, and 35% hydrazine hydrate aqueous solutions at 30$^{circ}C$ for various time intervals. The changes of properties such as weight loss, crystallinity, moisture regain, intrinsic viscosity, dyeability of acid dye, moisture permeability, and morphology were investigated on the aminolyzed fibers. The results obtained from this study were as follows. 1. Weight loss, increase in degradation, increase in crystallinity, decrease in intrinsic viscosity, and depression in melting point, etc. were found to increase gradually with increase of hydrazine hydrate treatment time. 2. Moisture regain and moisture permeability coefficient of aminolyzed fabrics increased in accordance with increasing the crystallinity of fibers. 3. Aminolyzed PET showed improved dyeability toward acid dye. 4. Chemical stress crack occurred on fiber surfaces in proportion to reaction time with hydrazine hydrate aqueous solution.