Growth of crack on the interface between lead-free solder ball and copper was monitored by means of the direct current potential difference method. The solder balls were jointed on the surface of a plate specimen of pure copper and cyclic tension test was carried out. The change in the potential difference of the solder ball increased with the number of cycles and the interface crack was initiated and grew. The relationship between the increase in the potential difference and the cracked area was evaluated and the results were compared with those obtained by the previous electric field analysis. Both results agreed with each other and this suggests that the interface crack can be monitored by the proposed method.