A high-strength, high conductivity copper alloy wire that is excellent in resistance to stress relaxation, which contains 1.0 to 4.5% by mass of Ni, 0.2 to 1.1% by mass of Si, 0.05 to 1.5% by mass of Sn, and less than 0.005% (including zero) by mass of S, with the balance being Cu and inevitable impurities, wherein the wire has a conductivity of from 20% to 60% IACS and a tensile strength of from 700 to 1,300 MPa, and a method of producing the same.