To investigate the effects of different factors, including sodium sulfate (Na2SO4), etc., on the formation of sulfur trioxide (SO3) during oxy-fuel circulating combustion, the experiment was performed to simulate the SO3 formation condition in this work. In the experiments, the heterogeneous formations of SO3 were measured using the controlled condensation method. The results showed that the effect of sodium sulfate on SO3 formation was significant. For different ash, at a high temperature, the SO3 concentration increased by 14.28-16.66 mg/m(3) compared to that of raw ash after adding Na2SO4. The main reason that sodium sulfate enhanced the SO3 formation may be that Na2SO4 can be combined with SiO2 and Al2O3 in ash to form NaAlSi3O8, (albite) and NaAlSiO4 (nepheline) with a low fusion temperature and then release SO3. In addition, the results show that increasing the SO2 concentration along with increasing the temperature is favorable for enhancing SO3 formation over the range of tested parameters.