The crystallinity of isotactic polypropylene UP), when deformed with plastic plane-strain compression, was studied with wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) techniques. A comparison of the obtained crystallinity data with annealed iPP samples was per-formed. The material used in this study was commercial iPP (weight-average molecular weight = 117,400 g/mol; number-average molecular weight = 17.300 g/mol). A significant decrease in the crystallinity was observed with increasing deformation pressure when the X-ray method was employed, whereas only a small decrease was registered when the DSC method of crystallinity determination was used. However, the annealed iPP Samples demonstrated a slight crystallinity increase when evaluated by both techniques. The reason for the difference between WAXS and DSC crystallinity results is discussed. This study of iPP specimens subjected to large deformation led us to the conclusion that the WAXS method provides accurate crystallinity values for the deformed material, whereas the values obtained by the DSC method do not reproduce the real crystallinity of the deformed material. This is due to the inherent heating process of the method, which causes a relaxation process and a significant change in the crystallinity of the deformed material, providing values nearer to its intrinsic equilibrium state.