Abstract: In order to improve the effective size and insertion loss of the substrate integrated waveguide (Substrate Integrated Waveguide, SIW) filter, a novel 1/8 mode substrate integrated waveguide (Eighth-Mode Substrate Integrated Waveguide, EMSIW) band-pass filter is proposed. Using 1/8 mode substrate integrated waveguide as cavity and using double-layer PCB structure, the filter can be reduced by more than 90% compared with the original size, and the edge effect of the filter is realized by slot coupling. Electromagnetic energy transmission can improve the insertion loss, and by designing the cross-coupling structure between cavities, a transmission zero is introduced at each side of the passband to improve its selectivity and out-of-band suppression level. The simulation results show that the center frequency of the filter is 3.9 GHz, the insertion loss is 1.06 dB, the effective size is only 9 mm×9 mm, and the out-of-band suppression is more than 40 dB in the range of 0~8 GHz spectrum. Physical processing is carried out to verify the conclusion, and the test results are in good agreement with the simulation.