Ce0.5Zr0.5O2 has been widely used in three-way catalyst (TWC). However, the thermal resistance is deteriorated at high temperature due to the phase separation of Ce0.5Zr 0.5O2. The phase separation and crystallite growth of Ce0.5Zr0.5O2 could be inhibited by adding Al ions. In this study, both the Ce0.5Zr0.5O2 and A12O3 doped Ce0.5Zr0.5O2 catalytic powders were synthesized via chemical co-precipitation method. The phase separation of Ce0.5Zr0.5O2 samples was observed as the calcination temperature was raised above 1100 °C. However, the A12O3 doped Ce0.5Zr0.5O 2 samples could maintain single phase till 1200°C. The Williamson-Hall method was employed on the X-ray peak profile of these powders to determine the variation of lattice strain. The phase separation of Ce 0.5Zr0.5O2 samples was observed due to the lattice strain induced by the occurrence of oxygen vacancies as the calcination temperature was raised to 1100 °C. The A12O3 doped Ce0.5Zr0.5O2 samples had similar phenomenon but the phase separation temperature was delayed to 1200 °C due to the diffusion barrier effect of Al ions.