Due to its extraordinary properties, graphene has been widely used as reinforcing nanofillers to enhance the mechanical properties of polymer- or metal-based composites. However, the weak interfacial interaction between the matrix and graphene is still a major bottleneck that considerably hinders its reinforcing effectiveness and efficiency. This study presents an atomistic study via molecular dynamics simulation on a chemical modification strategy where the aluminium (Al) substrate is modified with Al2O3 (with or without covalent bonds formed between Al2O3 and graphene) or Al4C3 to achieve significantly improved interfacial shear strength and overall mechanical properties of graphene-reinforced aluminium (Al/Gr) composites. Numerical results show that this strategy works very well and among the three cases considered, modifying Al substrate by Al2O3 without covalent bonds formed at the interface between Al2O3 and graphene produces the strongest interfacial interaction and the best mechanical properties. In the presence of covalent bonds, however, the reinforcing effect is adversely affected due to the sp2-sp3 bond transformation which partially degrades graphene. The present work provides, for the first time, valuable insight into the role of substrate surface modification on the mechanical performance of Al/Gr nanocomposites.