The modal loss factor η of a cantilever beam with magnetic rubber layer damping (MRLD) was obtained based on the derived equation of the same beam with conventional constrained layer damping (CLD) by equating the dissipation energy by MRLD to that by CLD in a vibrating cycle. The effect of damping layer shear modulus Gv on damping performance of the beam with MRLD was investigated. The results reveal that ,under a given displacement excitation W0, MCLD is the same as CLD for small Gv. Increasing Gv makes damping layers slide ,as a result, MRLD shows itself better damping than that of CLD in the beginning, whereas η decreases as Gv continues to rise until η of MRLD is less than that of CLD. Moreover, increasing W0 makes the valid region of Gv, for which MRLD exceeds CLD in damping property, move to the region of small Gv. So does the corresponding optimal shear modulus Gvopt for which the maximal damping can be obtained using MRLD.