Octadecylamine (ODA) and hyperbranched polyester (HBP) were successfully used to modify graphene oxide (GO) via chemical grafting, resulting in the preparation of modified GO (GO-O and GO-H). Both the modified and unmodified GO were dispersed in different solvents (water and toluene) and allowed to stand for three days to investigate their dispersibility in solvents of varying polarity. The results indicated that GO had strong hydrophilicity but poor hydrophobicity. Due to the introduction of alkyl chains on the GO surface, the dispersibility of GO-O in water deteriorated, while its dispersibility in non-polar solvents improved. Owing to the branched molecular structure and the presence of multiple hydroxyl groups in HBP, GO-H exhibited inferior dispersibility in both water and polar solvents compared to GO-O. Further, both the modified and unmodified GO were incorporated into epoxy resin (EP) to prepare a series of EP/GO composite materials, and the effects of GO on the curing behavior, mechanical properties, and dynamic mechanical properties of EP were comparatively studied. The results showed that with the addition of fillers, the curing temperature of the samples decreased, while the tensile strength, elongation at break, glass transition temperature, and storage modulus all increased. The improvement order was EP/GO-H>EP/GO-O>EP/GO. The addition of GO-H had a more significant impact on the properties of EP composites compared to GO-O. The tensile strength increased from 15.9 MPa for pure EP to 60.2 MPa with a 279% increase, the elongation at break increased from 10.5% for pure EP to 17.9% with a 70.5% increase, the storage modulus increased from 1 186 MPa for pure EP to 1 703 MPa with a 43.6% increase, and the glass transition temperature increased from 88.2 °C for pure EP to 100.8 °C.