We investigated the effect of lithium trifluoromethanesulfonate (LiCF3SO3) on the optical properties of poly (methyl methacrylate) (PMMA), known as a glassy transparent polymer. The absolute values of the stress-optical coefficients in both the glassy and rubbery states were found to be reduced by the addition of LiCF3SO3. Furthermore, the linear coefficient of thermal expansion of the blend containing 10 wt.% of LiCF3SO3 was lower than that of pure PMMA, suggesting that the birefringence caused by thermal expansion in laminated display systems would be greatly reduced. These results indicate that PMMA/LiCF3SO3 is an appropriate blend for optical films used for liquid crystal displays.