The wettability of a copper substrate by Sn–Ni alloy and the interfacial active properties of nickel were investigated. We determined the contact angle as a function of holding time in the Sn–Ni/Cu system at low Ni concentration. The contact angle of Sn can be decreased from 30° to 25° by adding 0.1 wt% Ni to Sn. We observed accumulation of the nickel at the solid–liquid interface. We assume that the nickel accumulation is caused by the interfacial active property of the nickel in an Sn–Ni/Cu system.