The monitoring for cracks of reinforced concrete(RC) bridge piers after ship collision plays a key role on the condition and safety evaluation of bridges. In this study, a crack monitoring approach using the stress wave measurement by piezoelectric ceramic(PZT) patches for a RC pier specimen under the freefallingweight impact was proposed. The impact test on a scale RC pier specimen with surface mounted PZT patches was carried out and the cracks of the specimen after impacts were detected using the variation of the amplitudes of the stress wave measurement under sinusoidal excitation and the wavelet packet energy of that under sweep signals excitations. The crack monitoring results agreed well with the test observation. The results showed that the proposed method could effectively monitor the crack damage of the pier structure after the impact.