Wide-angle X-ray scattering (WAXS) patterns of segmented poly(ether ester) fibers, which were made from poly(tetramethylene glycol) (PTMG) and poly(tetramethylene terephthalate) (PTMT), under tensile stress were studied to clarify the effect of the amorphous region on reversible crystal deformation of PTMT crystallites in the copolymers. The transition between the alpha and beta crystal phases was observed around 800 Kg/cm(2) in tensile stress with various copolymers. The transition was not observed in PTMG rich copolymers in which the amorphous region was too soft to bear this level of tensile stress.