The contribution of this study is to develop a miniaturised substrate integrated waveguide (SIW) cavity resonator with two evanescent‐mode resonances. The circuit area of the developed dual‐frequency SIW cavity is only 9.2% that of its conventional SIW cavity counterpart. A dual‐band filter is implemented from using two miniaturised SIW cavity resonators. The two‐passband frequencies can be independently controlled. Two transmission zeros are created by the two sides of each passband for enhancing the signal selectivity. A diplexer is also designed from using the proposed SIW cavities. A dual‐band filter and a diplexer are built for experimental verification. Agreements between measured and simulated data are observed. To the best of the authors’ knowledge, the designed dual‐band filter and diplexer have achieved the best circuit area efficiency in the categories of dual‐band SIW cavity filters and diplexers.