The properties of LSO-SDC composite electrolytes prepared by the mixed powder with different LSO/SDC mass ratios were studied. The apatite-type lanthanum silicates La10Si6O27（LSO） and Sm0.2Ce0.8O1.9（SDC） were synthesized via sol-gel process and glycine-nitrate process（GNP）, respectively. The phase structure, microstructure, relative density, thermal expansion properties and oxygen ion conductivity of the samples were investigated by means of techniques such as X-ray diffraction（XRD）, scanning electron microscopy（SEM）, Archimedes method, dilatometer, and AC impedance spectroscopy. The results showed that SDC addition to the samples could enhance the density of the samples. However, the LSO-SDC composite electrolyte sintered at 1550 oC was over sintering when the SDC content was 50 wt.%. At the lower content of SDC（0–10 wt.%）, the decrease of conductivity was predominantly attributed to the reducing concentration of carriers. However, the conductivities of the composite electrolytes increased with the increasing SDC content（10 wt.%–40 wt.%） because of the enhanced percolation of highly conductive SDC component in the microstructure of composite electrolytes. In addition,the dependence of conductivity on p（O2） showed that LSO-SDC composite electrolytes were stable in the examined range of p（O2）.
