The solid state amorphization of poly(ethylene terephthalate), applying mechanical milling, is described. Their effect on samples quenched or slowly cooled after the melt as well as in samples with different crystallinity degrees, attained from annealing treatments, is characterized using differential scanning calorimetry and X-ray diffraction at room temperature. In previously annealed poly(ethylene terephthalate) at several temperatures some differences appear in the amorphization after milling. However, effects other than the crystallinity degree have to be taken into account to explain them.