The dynamic birefringence and viscoelasticity were measured for poly(ethylene naphthalene-2,6-dicarboxylate), and the contributions from the main chain orientation and the rotational orientation of repeating unit to the strain-induced birefringence were evaluated. The values, together with published ones for other glassy polymers, were compared with the anisotropy of polarizability tensor of the repeating units. The contribution from the main chain orientation for various polymers was in accord with that evaluated from the excess polarizability of the unit in the main chain direction assuming a quasi-affine orientation of the unit. The contribution from the rotation was well correlated with the anistropy in the plane perpendicular to the chain axis. The degree of the rotational orientation was similar among the polymers. The rotational orientation was high for vinyl polymers with large side-groups.