Aluminum (Al) flake filled epoxy resin composites were fabricated, and the effect of flake surface treatment on the mechanical properties of the composites was investigated. The tensile shear test between the Al plate and epoxy resin displayed the highest interfacial shear strength of 12.1 MPa when the Al plate surface was treated with a silane coupling agent after cleaning with a degreasing agent. The highest tensile modulus 8.5 GPa in Al flake filled epoxy resin composite was obtained when the same treatment was applied to Al flake. However, the tensile strength of the composite was not improved. The aggregate structure of Al flakes was observed on the fracture surface of the specimen. The reason why the tensile strength of the composite was not improved was deduced that the shear force enough to deform Al flakes was not transferred from the epoxy resin to each flake due to the formation of Al flake aggregation.