To improve the compatibility of graphene nanoplatelets (GNPs) to polypropylene (PP), alkyl chains were grafted to the GNPs' surfaces by chemical reactions between alkyl amines and the corresponding functional groups of the GNPs. Hexylamine, dodecylamine, and octadecylamine were used respectively to prepare three different alkyl-grafted GNPs (alkyl-GNPs) with different alkyl chain lengths. Functional group analysis of the alkyl-GNPs by Fourier transform infrared confirmed the alkyl-grafting reaction. PP/alkyl-GNP nanocomposites with 0.5 phr of hexyl-, dodecyl-, or octadecyl-GNPs were prepared respectively by melt-blending and compression molding and their mechanical and thermal properties and fracture surfaces morphology were investigated. With increasing alkyl chain length the flexural strength and melting temperature increased marginally but the flexural modulus, impact strength and storage modulus of the nanocomposites increased to a large extent. POLYM. ENG. SCI., 59:752-756, 2019. (c) 2018 Society of Plastics Engineers