Amine degradation of poly(ethylene terphthalate) (PET) is interesting subject as a model for in-use ageing problems and as a way to investigate morphology of fibers and films. In order to investigate the structure and morphology of two 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 weight loss, crystallinity, crystallity size, and morphology were measured and observed on the insoluble fraction during amine degradation. The conclusions obtained from this study are as follows; 1. Weight loss, increase in crystallinity, and decrease in crystallite size were found to gradually increase with increased treatment time. 2. Initially ethylamine diffuses and permeates into PET fibers. in the second stage, decomposition and dissolution of PET fibers occurrs mainly in the amorphous regions. During the third stage, there is a gradual decrease in the reaction rate and this may the attributed to a relatively slow attack on crystalline regions in the sample. This indicates that amine degradation of PET fibers is a permeant reaction. 3. Stress crack occurred no fiber surfaces in proportion to reaction time with ethylamine solution, and finally this resulted in the breaking of PET fibers. 4. Change patterns of weight loss and crystallinity of PET fibers upon amine degradation were in accordance with those from the morphological study.