Graphene oxide-g-poly(ε-caprolactone) (GO-g-PCL) was synthesized and characterized in order to use it as a nanofiller for enhancing the toughness of poly(vinyl chloride) (PVC). Graphene oxide (GO) prepared from graphite was reacted with thionyl chloride to obtain acyl derivative of GO, and then GO-g-PCL was synthesized by grafting PCLs of three different molecular weights (530, 1250, 2000 g/mol) on it with various reactant ratios at 50 °C using a grafting-to method. Graft yield of the GO-g-PCL was obtained by TGA analysis comparing solvothermal treated GO. When the molar concentration of PCL per g GO was constant during the synthesis, increasing the MW of PCL reduced the number of PCL molecules grafted onto GO but increased the maximum graft yield. PVC/GO-g-PCL composite films were prepared by adding 0.3 wt% of GO-g-PCLs with a similar graft yield of 5.3-5.5% to PVC and their toughness changes were analysed. The composite films showed excellent increases in toughness compared to neat PVC, and the degree of increase was greater when the MW of PCL used for grafting was smaller.