The change of the relaxation process associated with the long-range motions during crystallization of an amorphous poly(butylene terephthalate)-polycarbonate block copolymer (PBT-PC) has been followed in real time by means of measurement of the dielectric complex permittivity. The observed decrease of the dielectric strength has been correlated to the appearance of crystallinity as detected by real-time wide- and small-angle X-ray scattering experiments using synchrotron radiation. The phenomenological Havriliak-Negami description has been used to analyze the changes of the dielectric strength, central relaxation time, and shape parameters describing the relaxation as crystallization proceeds. The influence of the crystalline lamellae on the dynamics of the amorphous phase for the PBT-PC(60/40) thermoplastic elastomer has been examined and compared with the relaxation processes of related homopolymers. The differences obtained are discussed by considering that PET block copolymers typically crystallize by formation of spherulitic superstructures in which the degree of crystallinity is smaller than in those formed by homopolymers. Finally, phase separation in the early stages of crystallization has been discussed on the basis of Cahn’s linearized theory of spinodal decomposition.