Highly oriented poly(trimethylene terephthalate) (PTT) fiber has a low birefringence that is unexpected for an aromatic polyester with a high refractive index. To explain this observation, the intrinsic birefringence Deltan(c)degrees of PTT crystal was calculated from its bond polarizabilities to be 0.029, This Deltan(c)degrees is almost an order of magnitude smaller than poly(ethylene terephthalate)'s value at 0.22, although both polymers have nearly identical crystal refractive indices. The small Deltan(c)degrees is due to the arrangement of PTT's methylene groups in gauche conformations, causing the chain-repeating unit to be tilted similar to53degrees away from the c axis toward the basal plane. Because of the small Deltan(c)degrees, the crystalline-phase orientation made only a small contribution to the overall birefringence despite the fiber's high crystallinity and orientation. To understand the effect of the number of methylene groups on polyester optical anisotropy, the Deltan(c)degrees's of a series of poly(m-alkylene terephthalates) with m = 2-5 were compared and correlated with psi: an angle made by the normal of the benzene ring with the crystal's axis. As psi' decreases, Deltan(c)degrees of the polyesters diminishes.