Abstract: An improved multifilament superconductor is fabricated by the method in which a tin rod or tin based alloy rod is clad with a copper based alloy tube, the thus obtained copper-clad rod is further clad with a niobium based alloy tube, and a copper material is applied as a stabilizer on an outer peripheral surface of the niobium based alloy tube to form a composite. This composite is then reduced and thermally treated at a temperature ranging from 640.degree. to 770.degree. C. to thereby form an Nb.sub.3 Sn superconductor composite with a filament tube therein. At least one of the copper based alloy tube and the niobium based alloy tube contains 0.1-5.0 at. % titanium, and in addition, the tin based alloy rod may contain 0.1-3.5 at. % titanium. The tin is contained in the copper-clad rod at weight % of about 25-85, preferably 30-80 weight %.