In this study, for the first time, a particle size effect on crystal Structure of Y2O3 particles was exploited to synthesize phase-pure monoclinic Y2O3 Particles. In the synthesis process, a precursor aerosol consisting of H-2 fuel gas and precursor droplets passed through an impactor before it entered a flame to form yttria particles. A round-jet impactor was used to remove the large precursor droplets. so that the product Y2O3 particles were all smaller than a critical size of approximately 1.5 mu m. Due to the particle size effect on crystal structure, the Y2O3 particles thus obtained were essentially phase-pure with the monoclinic structure. The result shows that, by using an impactor to alter the particle size distribution, it is possible to control the crystal structure of Y2O3 particles while maintaining relatively high synthesis yield. (C) 2008 Elsevier B.V. All rights reserved.