In this paper, we propose a novel microwave attenuator based on the multilayered substrate integrated waveguide (SIW). The large loss tangent of Flame Resistant-4 (FR-4) material in the Ku band is employed to design a highly lossy SIW resonator cavity as block element for the exploration of this attenuator. The relative permittivity and loss tangent of FR-4 at high frequency are at first experimentally extracted by testing a fabricated ring resonator on FR-4 substrate. Then the characteristics of single unit of this attenuator versus a few critical geometrical dimensions are studied. Next, an equivalent circuit of this single-unit structure is developed to theoretically predict the propagation characteristics of the designed attenuator in an efficient and effective manner. Finally, an example of SIW-based attenuator is designed and measured. The measured attenuation factor agrees with the simulated and predicted ones in the stopband frequency of our concern. In addition, good impedance matching at input and output ports is satisfactorily realized.