Polyester fiber treated with aqueous solution of guanidine carbonate and sodium hydroxide mixture has amide bond and amine end group owing to both hydrolysis by sodium hydroxide and aminolysis by guanidine in ester bond of polyester. The mechanism of weight loss of PET and reactivities of guanidine carbonate and sodium hydroxide to PET were qualitatively investigated by using infra red spectrum and quantitatively by using nitrogen content and the amount of acid dye absorbed. When polyester fiber was treated with aqueous solution of guanidine carbonated and sodium hydroxide mixture, weight loss, density and crystallinity of PET and the amount of amine end group and acid dye absorbed suddenly increased and intrinsic viscosity decreased according as the concentration of guanidine carbonate at constant concentration of sodium hydroxide or that of sodium hydroxide at constant concentration of guanidine carbonate became larger. From these results, we can assume amorphous region is more selectively degraded than crystal region, so degradation reaction occurs in the surface as well as in the interior region of PET fiber when it is treated with aqueous solution of guanidine carbonate and sodium hydroxide.