In this study, in order to investigate the physical properties of 2 kinds of PET fibers(i.e., shaped and circular ones), PET fibers have been degraded in 50 and 60% aqueous ethylamine solutions at 30$pm$1$^{circ}C$ for various time intervals. The changes of such properties as tensile property, dynamic viscoelasticity, and thermal behavior were measured on the in soluble fraction during amine degradation. The conclusions obtained from this study were as follows; 1. Decrease in specific stress and breaking extension, decrease in the intensity and temperature of the maximum tan$delta$ peak, and depression in melting point, etc. were found to gradually increase with increased ethylamine treatment time. 2. Both intensity and temperature of maximum tan$delta$ peak decreased during the initial course of aminolysis. These phenomena could be ascribed not only to the decrease of the amorphous regions but also to the decrease of the lateral order, due to chain scission in low order regions. 3. Melting point decreased with the decrease of crystallite size. The double melting peaks were observed for the aminolyzed fibers, suggesting that 2 kinds of different fine crystal structures existed in PET fibers. 4. Change patterns of specific stress and breaking extension, maximum tan$delta$ peak, and thermal behavior of PET fibers upon amine degradation were in accordance with those from the morphological study.