Crystalline modifications a and beta of uniaxially oriented poly(tetramethylene naphthalate) [PTMN] sample have been found to show a reversible phase transition, which is induced by an application of a tensile force along the draw axis, as clarified by the measurement of polarized infrared spectra under tension. The transition behavior was found to be essentially the same with that observed for uniaxially oriented poly(tetramethylene terephthalte) [PTMT] sample. The beta phase fraction, evaluated by quantitative analysis of the infrared spectra, was found to be linearly proportional to the strain and increase dramatically beyond a critical stress. The stress-strain curve measured for the bulk PTMN sample showed a plateau region, in which the alpha-beta transition occurred. The plateau length became longer by raising the temperature of heat treatment of the samples or with increasing the degree of crystallinity. All these characteristic features are similar to those observed for PTMT. But, in the PTMT case, the plateau stress was almost constant irrespective of the annealing temperature. The stress-strain curve of the PTMT sample could be interpreted reasonably on the basis of a mechanical model in which alpha crystal and amorphous region are connected in a series mode. On the other hand, the plateau region in the stress-strain curve of the PTMN sample was found to shift apparently toward higher stress side and became more difficult to detect for the sample annealed at lower temperature. This was due to higher residual strain or beta fraction remained in the sample. High-temperature annealing accelerated a stress relaxation, causing a recovery of the original a form and making the plateau part clearer. All these behaviors could be interpreted reasonably by employing a mechanical model in which a serially connected part of beta crystal and amorphous region is combined in a parallel mode with a serial part of crystal and amorphous region. The contribution of the beta crystal is higher for the sample prepared at a lower temperature.