In this paper, the effects on the mutual coupling of PMC/PEC parallel plates at millimeter-waves are studied and presented. The PMC/PEC parallel plates are mainly used to reduce the mutual coupling between two aperture coupled cylindrical DRAs excited in their fundamental modes HEM 11δ . The DRAs are separated by 0.5 λ and operated at millimeter-waves. The PMC/PEC plates are composed of a PEC layer and a PMC layer. Using this approach, a minimum mutual coupling reduction of 15.5 dB is achieved at 64 GHz. Furthermore, a superstrate is also used to increase the gain for compensating the path losses and oxygen absorption at 60 GHz. Simulations of S parameters and gain are obtained using HFSS and confirmed the principle of mutual coupling reduction by the proposed PMC/PEC parallel plates. This approach can be used in DRA arrays for millimeter-waves MIMO applications.