Cyclic olefin copolymer(COC) is one of the new class of polymers based on cyclic olefin monomer and ethylene. Because of the bulky cyclic olefin units randomly or alternately attached to the polymer backbone, the copolymer becomes amorphous and shows properties of high glass transition temperature(Tg), optical clarity, low shrinkage, low moisture absorption, and low birefringence. Although the COC has many desirable properties as a potential engineering plastic, the disadvantage of low impact strength of COC remains still problematic. Therefore, an improvement of impact strength of COC is very essential to expand its applications. Also, it has been demanded to further improve the impact strength with maintaining an excellent transparency of COC. In this study, the toughening of COC(ethylene-trtracyclododecene copolymer) with styrenic elastomer while maintaining its optical transparency was investigated. SEBS(Polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene) having 58% styrene block is selected as a impact modifier because of its closely matched refractive index to COC. It is found that the impact strength of COC/SEBS blends strongly depends on the dispersed particle size and SEBS content. When the SEBS content is 10wt, the impact strength of COC/SEBS increases with decreasing the particle size and shows a sharp transition at about 0.5μm particle size. And it is observed that the optical transparency of COC/SEBS blends is strongly influenced by the refractive index difference between SEBS and COC. The optical transparency of COC/SEBS blends is improved as the relative refractive index approaches to unity. The control of the refractive index of SEBS by adding the polymer which has a miscibility with each segment in SEBS is studied. The refractive index of the dispersed SEBS can be adjusted by adding a PS and EPR and it leads to more closely matched refractive index to COC. From the morphological observation, the multi lamella vesicles are observed in the cast films prepared by solution blending due to the lamella microstructure of SEBS itself.