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This study proposes a highly sensitive sensor based on localised spoof surface plasmons (LSSPs) for omni‐directional cracks inspection in metal surfaces. A defected ground structure based on the complementary metallic spiral structure (CMSS) is embedded on the ground of a microstrip line to excite LSSPs, and the mode characteristics of these LSSPs are studied. Then an equivalent lumped circuit method is presented to reveal the operation mechanisms behind the trapped LSSPs. Due to the strong field confinement, LSSPs are used as a sensor to inspect cracks in a metal surface, where a 0.2‐mm‐wide and 2‐mm‐deep crack yields a 400 MHz resonance frequency shift. Meanwhile, a big resonance frequency shift is also obtained for omni‐directional detection of surface cracks because of the rotational symmetry of the CMSS. It indicates that the proposed sensor is orientation independent, so that any cracks would not be missed in the testing and cracks would be located more easily. Therefore, the proposed sensor has more potentials than other sensors in actual practice.

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