A relay gain assignment method and a relay gain assignment device are provided. The method and the device are performed in a MIMO relay system comprising M spatial degrees of freedom and L relay terminals, wherein M and L are positive integers. The relay gain assignment method comprises the steps of: (a) defining a system model for the MIMO relay system, wherein the system model comprises a relay-connected noise and a destination-connected noise; (b) calculating power of the relay-connected noise and power of the destination-connected noise; (c) determining the dominating one between the power of the relay-connected noise and the power of the destination-connected noise; (d) defining a proper noise dominating model based on the determination result of the step (c); (e) calculating L relay gains for the L relay terminals; and (f) assigning each of the relay gains to the corresponding relay terminal of the MIMO relay system.