The crystallinity (x(c)) of high-pressure-crystallized poly(ethylene terephthalate) (PET), which shows values of x(c) congruent to 0.90 when measured by differential scanning calorimetry (DSC), was examined by means of wide-angle X-ray scattering (WAXS). PET samples crystallized under normal conditions were also investigated by WAXS, DSC, and density for comparison. It is shown that the degree of crystallinity derived from WAXS for the high-pressure-crystallized samples is smaller than the values obtained by calorimetry and density. However, the crystallinity values from WAXS are still larger than the values found in the material crystallized under normal pressure. Furthermore, it is shown that the unit cell dimensions of the PET samples are not affected by the pressure during crystallization. The lower value of the degree of crystallinity as measured by WAXS is discussed in terms of an interface of finite thickness between the crystals and the amorphous regions.