The thermal expansion and conductivities have been investigated for Co3+ doped lanthanum silicates. The apatite-type lanthanum silicates with formula La10Si6-xCoxO27-x/2 (x = 0.2, 0.4, 0.6, 0.8, 1.0, 1.5) were synthesized by sal-gel process. The thermal expansion coefficient (TEC) of La10Si6-xCoxO27-x/2 was improved with increasing cobalt content because of the lower valence and larger radius of Co3+ ion compared to Si4+. Analysis of AC impedance spectroscopy showed that conductivity increased first and then decreased with increasing cobalt content. There is an optimum doping amount of cobalt and La10Si5.2Co0.8O26.6 exhibits the highest conductivity of 3.33 x 10(-2) S/cm at 800 degrees C. When x <= 0.8, the local distortion caused by doping with Co3+ can significantly affect the oxygen channels and assist the migration of the interstitial oxide ions, resulting in the improvement of ionic conductivity. However, excess Co3+ dopant (0.8 < x <= 1.5) reduced the number of interstitial oxide ions and decreased the conductivity. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.