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For the first time in this study, a comprehensive experimental investigation was performed to investigate the effect of different roller diameter ratios ( Rd) on mechanical properties and plastic instability of Al/Cu composite produced by the asymmetric accumulative roll bonding process. For this purpose, Al/Cu composites were produced using three different Rd in three passes. Then, bond strength, mechanical properties, microstructure, and fracture surface of all specimens were determined. The results showed that due to the mechanisms underlying the severe plastic deformation processes, all composites’ mechanical properties sharply increased compared to aluminum and copper. As the Rd increases, the bond strength and hardness of the Al/Cu interface increase due to improved plastic deformation rate, surface expansion, and more extrusion of the material at the boundary of two layers. Besides, the hardness along the thickness was found to be very heterogeneous due to the uneven distribution of strain and strain hardening. Scanning Electron Microscopy (SEM) micrographs depicted a decrease in the layers’ thickness, an increase in the plastic instability of the copper, and improved bonding strength of the layers as Rd rises. However, in all specimens, the layered structure was maintained. Fracture micrographs also revealed that the fracture mechanism is ductile for both aluminum and copper layers at different Rd. However, with increasing the Rd, the number and the depth of the dimples along with micro-pores decrease. Finally, the non-uniform strain distribution of the process caused the micro-dimples to be drawn in different directions.