The effect of temperature and complex dopant composition on oxygen ion conductivity in solid oxide electrolyte fuel cells was investigated by atomistic molecular dynamics simulation. A new electrolyte (Nd2-xGdxZr2O7) was selected to study oxygen ion conductivity using three Gd compositions (x = 0.8, 1.0, and 1.2) in a wide range of temperature (T = 1273 K-1873 K). MSD results of cations showed these groups of electrolyte are stable at high operating temperature. The first composition (x = 0.8) had the highest ionic conductivity that was in good agreement with the experimental data. A simple effective model that works with configuration energy of the oxygen crossing plate was applied to explain the observed conductivity trend. The model illustrated the point as well. Increasing Gd concentration decreases existence probability of easy crossing plate. Radial distribution function analysis also confirmed results. Thermal expansion of the electrolyte has a major effect on the selecting of the electrolyte materials; thus, this important factor was also studied. Results showed the first composition had the greatest thermal expansion. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.