The thermal expansion coefficients of CeO2 and Gd-doped ceria were measured in the range from 100 to 873 K and those of reduced ceria were measured from 100 to 323 K. The thermal expansion coefficient of Ce1-xGdxO2-x/2 became large with increasing the dopant content, x. The thermal expansion coefficient of reduced ceria became large with increasing the vacancy content of oxygen. These increases of the thermal expansion coefficients result from weakening binding energy due to the increase of oxygen vacancy. The thermal expansion coefficients of CeO2 and Gd-doped ceria were calculated theoretically and they were in good agreement with experimental ones except for the higher temperature range.