A new sensor based on complementary split-ring resonators (CSRR) is proposed. The sensor can detect sub-millimeter micro-surface cracks effectively. The crack perturbs the electromagnetic field around in the close vicinity of the CSRR gap, thereby causing a shift in the resonance frequency of the CSRR resonator. The sensor is etched-out in the ground plane of a microstrip-line using printed circuit board technology, thus making it significantly less expensive than crack detection sensors based on waveguides. The sensor presented here is designed to operate around 7 GHz. The sensor exhibits a frequency shift of more than 240 MHz for a 100 µm crack.