The PZT-based electro-mechanical (E/M) method was utilized to measure the impedance of a 1-D steel beam with cracks introducing and developing. Quantitatively assessment of cracks was made by using damage metrics Root Mean Square Deviation (RMSD) and Covariance (Cov). Changes in the coupled impedance of PZT and the beam (500mm×20mm×6mm) were investigated during the appearance and extension of cracks. With the depth of crack dc increasing from 0.6mm to 1.8mm, the peaks of impedance spectrum shifted to lower frequency in the frequency range of 182 kHz-370 kHz and the amplitude showed a decreasing tendency. Moreover, the initial crack brought more significant changes in impedance than that from the developing of cracks. The relationships between dc and RMSD/Cov were investigated. It was proved that RMSD and Cov were efficient in quantitatively presenting the extension of cracks on the 1-D beam.