Dynamic viscoelasticity and dynamic birefringence measurements were performed on a quenched amorphous sample of a syndiotactic polystyrene, sPS. The measurements were carried out at temperatures where crystallization did not occur above the glass transition temperature. The data were analyzed with a modified stress-optical rule: The complex Young＇s modulus was separated into two components, designated with subscripts R and G, for which the ordinary stress-optical rule held well individually. A comparison with the results for a conventional amorphous atactic polystyrene has shown that the stereoregularity of sPS enhances the stiffness of main chain and then increases the optical anisotropy of segments. This conclusion is in accord with theoretical calculation on coil dimension and optical anisotropy.