The electromigration of contacts in shallow junction devices is a reliability issue for the future very large scale integration technology. The stability of silicide contacts against a high current density is unknown. We have observed a strong polarity effect of the electromigration-induced failure at both Ni/Ni2Si/n+-Si and Ni/Ni2Si/p+-Si contact pairs. They were found to fail preferentially at the cathode. The unreacted Ni/n+-Si and Ni/p+-Si contact pairs have also been studied. The latter fails at the cathode while the former fails at the anode. Failure mechanisms are proposed to explain the polarity effects.