La9.33Si6O26 (LSO) nanocrystals, quasi-spherical in shape and of an average size ca. 45 nm, were synthesized via a solvothermal process and used to mix with Sm0.2Ce0.8O1.9 (SDC) ultrafine powder to fabricate SDC-LSO composite electrolytes for the applications in IT-SOFCs. The microstructures and phase components of LSO nanocrystals and SDC LSO composite electrolytes were characterized by XRD, TEM and HRTEM and, in particular, the sintering performance and oxide ion conductivity of SDC LSO composites with different SDC/LSO volume ratios were studied. It has been found that the LSO nanocrystals are homogeneously dispersed in the as-sintered SDC LSO composites and no impurity phases due to chemical reactions can be detected between SDC and LSO particles by XRD, but the sintering performance is remarkably improved with a temperature reduction by 100 -250 degrees C, compared to that for the individual constituent phases. Moreover, the oxide ions conductivity of SDC-LSO composites can be conspicuously enhanced with the sample SL7525 (SDC/LSO = 0.75/0.25) showing the highest enhancement by 118%, i.e. 1.62 times that of SDC at 800 degrees C. The SDC/LSO hetero-interfaces with high energy and appropriate residual thermal stresses in the SDC-LSO composite microstructures are considered responsible for their improved sintering performance and significant enhancements in oxide ions conductivity.. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.