A method for building the phase diagram of syndiotactic polypropylene in stretched fibers as a function of stereoregularity, degree of deformation, and temperature is reported. The method is based on the analysis of the X-ray fiber diffraction patterns of samples of different stereoregularity, prepared with different metallocene catalysts and stretched at different degrees of deformation and stretching temperatures. For all the analyzed samples the boundary lines between the different regions of stability of the different polymorphic forms have been determined by the emergence or disappearance of X-ray reflections typical of the different forms. Although the phase diagram is given from a dynamic rather than a thermodynamic perspective, the polymorphic forms observed by X-ray diffraction correspond to the equilibrium modifications, thermodynamically stable at those values of stereoregularity and deformation.