To investigate structural factors, necessary to obtain a valuable industrial fiber possessing excellent thermomechanical properties, poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) fibers were produced by high-speed melt-spinning to a take-up speed of 8 km/min, followed by low-temperature annealing between the glass-transition temperature (T-g) and exothermic cold crystallization temperature (T-c cold) where little transition of crystalline phase, as well as little thermal degradation, takes place. Their thermomechanical behavior, as well as structural variations, were investigated through differential scanning calorimetry, Rheovibron, thermomechanical analysis (TMA), and tensile testing. Two types of the alpha- and alpha'-dispersions were observed at near T-g and at a temperature 50-60degreesC higher than T-g, respectively. The dispersions were affected by rearranged structures, which are generated by developing an inhomogeneous taut structure with rigidity of aromatic segment and aliphatic segment. The alpha-dispersion seemed to reflect an inhomogeneous taut structure by the less nearly arranged segments. Consequently, at intermediate take-up speeds between 2 and 6 km/min the inhomogeneous taut structure may be partially formed, but the homogeneously ordered structure may be enlarged as the take-up speed and annealing temperature increased. Thermal shrinkage increased above the alpha-dispersion temperature, which suggested that the onset point of dimensional change in PEN fibers was attributed to alpha-dispersion. In the case of annealed fibers, the start of length change coincided with the respective annealing temperatures, which indicated that dimensional stability could be gained from restraining the inhomogeneous taut structure in the amorphous region without the transition of crystalline phase by annealing between T-g and T-c cold. Therefore, to obtain dimensional stability in PEN fibers, it is supposed that the inhomogeneous taut structure exhibited by the alpha-dispersion should be controlled. (C) 2004 Wiley Periodicals, Inc.