The effect of aqueous ammonia treatment on th physical properties and structure of PET tire cord fiber having different long periods was investigated. It was found that degradation occurred mainly in the amorphous region in the initial stage, and then occurred in the crystalline region. The tenacity decreased with increase in aqueous ammonia treatment time and temperature. Molecular relaxation seems to be a dominant process during the initial stage of aqueous ammonia treatment. As the treatment time increases, chain scission appears to occur remarkably at the later stage. Aqueous ammonia treatment exhibited stronger effects on the change of physical properties and fine structure of a sample having a smaller long period than on samples having larger long periods. From these results, it is found that the chemical treatment of PET tire cord is quite different from the result observed through physical treatment (repeated extensional fatigue) in physical properties and structure.