The solid electrolytes of Ce0.9-xYxCa0.1O2.delta with x = 0 and 0.05 were prepared by mechanical milling technique and investigated its dielectric properties. The frequency dependent ac conductivity of this oxide-ion conductor showed an unusual behavior compared to that of the cation conductors. As temperature increases the oxygen vacancy concentration increases due to the dissociation of oxygen vacancies from the defect pairs. The co-doped sample exhibit slightly higher conductivity compared to the singly doped sample. Dielectric relaxation analysis exhibited two relaxation peaks as a result of the bulk and grain boundary ionic migration process. Temperature dependent resonance frequencies and tan delta maxima of the relaxation peaks are used to measure the migration energy E-M supporting the O2- migration and dissociation energy E-D for the creation of free oxygen vacancy from the defect associates. A single relaxation peak observed in the modulus analysis corresponds to the reorientation relaxation of the defect associates.