The complex preparation processes of GO/polymer composites are a limitation for their application. In this study, epoxy composites filled with in situ functionalized graphene oxide (GO) were prepared by a facile wet mixing and solvent evaporation method. The structure and composition of 3 ',4 '-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (2021P)-functionalized GO (GO-2021P) were characterized. With surface modification, GO became more thermally stable and changed its nature from hydrophilic to hydrophobic. The influences of surface functionalization on the morphology of GO, its dispersion in the epoxy matrix, the mechanical and thermal behavior of the composites were fully investigated. With grafting of 2021P, the dispersion/exfoliation status of GO and interfacial interaction between GO and epoxy matrix were much improved. For epoxy composite with 1.0 wt% GO-2021P, the tensile strength and Young's modulus increased by 34.0% and 25.9%, respectively. Increased glass transition temperature and thermal stability were also observed by dynamic mechanical analysis and thermogravimetric analysis. High-performance functionalized GO/epoxy composites can be produced using this technique.