The structure and morphology of biaxially stretched poly(ethylene terephthalate) (PET) film treated with ethylenediamine aqueous solution were investigated by means of weight loss, surface morphology, crystallinity, melting point, acid dye adsorption and degree of degradation measurements. The PET film was degraded with 40%, 50%, and 60% ethylenediamine aqueous solution at 6$0^{circ}C$. Because of the increased chemical stress cracking by residual stress of skin layer, the rate of degradation of skin layer was faster than that of core phase. Owing to this, separation of the skin layer from the core phase was possible. Amount of dye adsorbed on aminolyzed PET film was linearly correlated with the crystallinity and the degree of degradation, suggesting that the degradation occurs at the interface between crystals. The melting point was increased in the early stage of aminolysis, but decreased when more than 30% weight loss occurred. From this fact, it is concluded that PET chains in crystalline region were disintegrated by passing of time.