A solid electrolyte for an oxygen concentration sensor is provided by mixing ZrO.sub.2 and about 6.5 to about 8% by mol of a stabilizer such as Y.sub.2 O.sub.3, grinding the mixture to give a BET specific surface area of about 9 to 20 m.sup.2 /g, adding to the ground mixture a forming aid, forming the mixture into a predetermined configuration and firing the formed mixture at about 1420.degree. to 1570.degree. C. The electrolyte has a surface-average grain size of about 5 microns or less and a structure of a cubic phase alone at the surface thereof and, furthermore, may contain about 5% or less of a monoclinic phase per the total of the cubic and monoclinic phases, in a finely ground state.