Graphene oxide-g-poly(ε-caprolactone) (GO-g-PCL) with various graft yields was synthesized by grafting PCL diol on acyl derivative of GO and used as a filler to enhance the toughness of poly(vinyl chloride) (PVC). PVC/ GO-g-PCL composite films were prepared by mixing GO-g-PCL of different graft yields with PVC at a constant content or by mixing GO-g-PCL of a constant graft yield with PVC at different contents. Then changes in physical properties including toughness of the composite films and the interactions between PVC and GO-g-PCL were analyzed. GO-g-PCL was more effective in enhancing toughness of PVC than GO, and the toughness improved more as increasing the graft yield of GO-g-PCL. When GO-g(6.4%)-PCL was used, the toughness improvement effect was greatest when the content was 0.3 wt%, and when it was 0.5 wt% or more, the toughness of the composite decreased. GO-g-PCL improves the toughness of PVC by loosening the PVC molecular chain, but when the content exceeds a certain level, the toughness decreases because they aggregate together. Even when GO-g-PCL and bis(2-ethylhexyl) phthalate plasticizer were used together, the effect of improving PVC toughness by the addition of GO-g-PCL was maintained.