The structural development of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) fibers was analyzed by in situ X-ray diffraction and fiber temperature measurements. The PEN fiber was drawn continuously under three drawing stresses, where the neck-drawing point was fixed accurately by CO2 laser irradiation heating. The developed crystal structures of the drawn fibers depended on the drawing stresses, that is, only the alpha-crystal was obtained under a drawing stress of 148 MPa, an alpha-rich mixed crystal was obtained for 54 MPa, and a beta-rich mixed crystal was obtained under 23 MPa stress. Fiber containing over 70% beta-crystal was obtained in the third case. Orientation-induced crystallization rates (K) and crystallization induction times (t(0)) were estimated for the three drawing stresses: K = 2210 s(-1) and t(0) = 0.5 ms for 148 MPa, K = 940 s(-1) and t(0) = 1.0 ms for 54 MPa, and K= 655 s(-1) and t(0) = 4.0 ms for 23 MPa. In addition, the drawing stress acted as a definitive influence not only on the resulting crystal form but also on the chain conformation of the mesophase structure. The d-spacing of the (001') diffraction increased with drawing stress, and the longer (001') spacing generated the alpha-crystal while the comparatively shorter (001') spacing yielded the beta-crystal. The d-spacings of 1.27 and 1.23 nm for the drawing stresses of 148 and 23 MPa, respectively, were somewhat shorter than the c-axis lengths of the alpha- and beta-crystals of 1.32 and 1.27 nm, respectively. (C) 2012 Elsevier Ltd. All rights reserved.