Y and Sr co-doped ceria (Ce0.8 + xY0.2 -2xSrxO1.9) samples which have the fixed oxygen vacancies concentrations, were synthesized by a gel combustion method followed by the common ceramic preparation process, and the effect of co-doping on the ionic conductivity was investigated. XRD results confirmed cubic fluorite structure in the samples. The lattice constant increases with increasing Sr content, and different strains of the compressive strain and the tensile strain exist in different samples. The sintered samples with the relative density of about 92% showed well defined grains, good densification with few closed pores. The activation energies of co-doped ceria decreased, reached a minimum and then increased with the increasing of Sr2+ concentration. The ionic conductivity of co-doped ceria increased with increasing Sr2+ doping concentration, reached a maximum and then decreased at higher doping level. The variation of the ionic conductivity and the activation energy with increasing Sr2+ doping concentration can be attributed to the different strain state in samples. The activation energy reached a minimum value and conductivity reached a maximum value for YSDC-05. The conductivity of YSDC-05 at 600 degrees C, 700 degrees C and 800 degrees C reached 0.0058 S.cm(-1), 0.0088 S.cm(-1) and 0.0099 S.cm(-1), respectively. (C) 2016 Elsevier B.V. All rights reserved.