Composite electrolyte made of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM) and a mixture of Li₂CO₃/Na₂CO₃ (48/52 at.%) was investigated with respect to their structure, morphology and ionic conductivity. Several proportions of polymethyl methacrylate (PMMA) was introduced during preparation of macro-porous LSGM matrices to create interconnected pore structures with different porosity factor that were found conducive for carbonate infiltration and integration with the membrane materials. Both mercury intrusion method and drainage method were used to investigate the porosity factor of LSGM matrices. The structure and morphology of composite electrolyte were studied by SEM. The ac impedance method was used to test the ionic conductivity. The conductivity of composite electrolytes were increased with the increasing proportions of PMMA after the melting point of carbonate and reached 0.088 S/cm while the proportion of PMMA was 40 wt.% at 600 oC. A watershed can be found from the Arrhenius plots [ln(σ.T) vs T_-1] of composite electrolytes at about 480 oC indicating the shift of the conduction mechanism caused by the melting of carbonate.