Oxidation resistant effects of AgS in a ternary Sn-Ag-Al solder were investigated from the aspect of electrical conductivity. Combined reactions, associated with combustion and ionization of AgS, contributed to the removal of the oxide layers of the AgS-containing solder; as a result, lowering the overall O concentration. Because the diffusion of AgS suppressed oxidation in the Sn matrix, the electrical resistivity was significantly decreased. With regard to the improvement of the oxidation resistance and electrical conductivity, these oxidation resistant functions of AgS also inhibited the formation of intermetallic compounds (IMCs) and interrupted the growth of Sn whiskers.