The alkali hydrolysis of poly(ethylene terephthalate) and its copolyester were investigated at various conditions of alkaline concentration, time and temperature. Loss in weight increased with alkali concentration. time and temperature. but was not changed in presence of ethylene glycol added in bath. In comparison with regular polyester, easy shrinkable copolyester showed rapid hydrolysis rate due to its loose physical structure. As conjugated polyester was hydrolyzed by alkaline solution. the remained regular polyester component became fine and was hydrolyzed more rapid than the regular polyester. The activation energy of alkaline hydolysis of easy shrinkable polyester and regular polyester were low (about 10-20kcal/mol). This means that there is no penetration of alkaline solution into the inner space of fiber. and the hydrolysis is limited only on the fiber surface.