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Ultrafine Y2O3 powder was produced from metallic yttrium powder by a radio frequency (RF) plasma method, and used as an additive to aluminium nitride in normal sintering. The average particle size (Microtrack method), shape (TEM), specific surface area (BET method) and crystalline structure (X-ray diffraction) of the ultrafine Y2O3 powder were evaluated. Density and thermal conductivity were measured for the sintered aluminium nitride containing the ultrafine or commercial Y2O3 powders as additive. Grain boundaries of the sintered aluminium nitride were also examined by X-ray diffraction and SEM. Yttrium distribution in the sintered aluminium nitride was analyzed by EPMA. The results are summarized as follows:(1) The ultrafine Y2O3 powder contained two allotropic forms. The average particle size and specific surface area of the ultrafine Y2O3 powder were 1.63μm and 14.2m2/g respectively. Particles were spherical. One of the commercial powders tended to form agglomerates, and had a higher specific surface area with bigger effective particle size.(2) The aluminium nitride powder was densified with the ultrafine Y2O3 powder more extensively than the one of the commercial Y2O3 powders at 1700°C.(3) Thermal conductivity of the sintered aluminium nitride with the ultrafine Y2O3 powder was higher (120W/(K·m)) than that with the commercial Y2O3 powders (about 50W/(K·m)). Both of them were sintered at 1700°C.(4) Yttrium ions of the sintered aluminium nitride with the ultrafine Y2O3 powder were distributed throughout the sintered bodies compared with the one with the commercial Y2O3 powders.(5) The sintered aluminium nitride with the ultrafine Y2O3 powder shows a lower oxygen content than the one with the commercial ones.