In vacuum chambers and accelerating cavities in recent accelerators, RF absorbing materials are widely used for damping higher-order modes and reducing the coupling impedance. For precisely measuring the RF characteristics of absorbing materials, we developed a new resonant-cavity method for measuring RF absorbing materials. In our method, the cavity-eigenmode simulation is used to evaluate the complex permittivity or permeability of a sample from the results of RF measurements. We fabricated a rectangular cavity for this method and measured the complex permeability of a ferrite sample in frequency range of 1.5–3 GHz. The result was compared with one from the Nicolson-Ross-Weir method.