X-ray powder diffraction and molecular modeling are used to identify the crystal structure and chain conformation of poly(trimethylene 2,6-naphthalate) (PTN) beta-form crystal. The unit cell of PTN beta-form crystal was determined to be a triclinic with dimensions of a = 0.4665 nm, b = 0.7014 nm, c = 2.2177 nm, alpha = 100.85degrees, beta = 88.78degrees and gamma = 120.63degrees, and the space group of the crystal is identified as P (I) over bar. The observed crystal density of 1.37 g cm(-3) and the determined dimensions of unit cell indicate that the unit cell contains one polymer chain with two repeating units. In the unit cell, each trimethylene unit in PTN backbone is in gauche/gauche conformation and neighboring naphthalene units are in face-to-face type arrangement, forming pi-stacks that lead to the lowest energy of the unit cell. (C) 2003 Elsevier Ltd. All rights reserved.