Samarium and neodymium co-doped ceria (Ce0.8Sm0.1Nd0.1O2-delta abbreviated SNDC) and lanthanum silicate (La9.33Si6O26 abbreviated LSO) composite electrolytes with different mixing ratio were prepared at 1500 and 1550 degrees C. The composite obtained were all with high density, and no chemical reaction between two phases was observed during sintering according to the XRD result. The addition of LSO has a great influence on the suppression of grain growth of SNDC, and brought about the change in conduction behavior of composite. High distortion at hetero interface has been observed by TEM. The composite with 30 wt% LSO exhibits the highest conductivity (0.0036 S/cm at 500 degrees C) with an improvement of 37% compared with SNDC. The measurement at low oxygen partial pressure demonstrated the high chemical stability of composite electrolyte, making it a promising candidate for intermediate temperature SOFCs or oxygen sensors.