PZT piezoelectric ceramic was used as sensing element to fabricate piezoelectric sensor. The fabricated PZT piezoelectric sensor was embedded into and affixed to the structure to detect the structural damage, respectively. The structural crack damage was investigated using the impedance spectra of the sensor. The results show that the electric impedance of the PZT piezoelectric sensor with both different arrangements can reflect the variations of the structural crack damage in the testing frequency ranges. In the frequency range of 20-70 kHz, both the impedance value and the resonance frequency of the embedded PZT piezoelectric sensor can show the incipient crack damage and the increase of crack depth clearly. The impedance spectra of the affixed and embedded PZT piezoelectric sensors show the similar variation regularity with increasing the structural crack damage degree. Furthermore, the impedance spectra variation of the PZT piezoelectric sensor with different arrangements in the thickness resonance frequency range is more obvious than that in the planar resonance frequency range. A scalar damage metric is presented based on the impedance spectra of the PZT piezoelectric sensor around the resonance frequency, and the cracks in different positions of the structure were also analyzed using the damage metric. The variation of structural crack damage can be observed effectively and obviously using this damage metric.