Structural changes of Y₂O₃-SiO₂ solid system during a fine milling in air were investigated. The milling of a mixture in a molar ratio Y₂O₃/SiO₂ = 7/9 affect the structure of the solids. After heating the samples at T > 1000°C new phases were obtained by a thermal reaction between Y₂O₃ and SiO₂, as a result of the mechanochemical activation. The extent of the mechanochemical reaction was higher when using hydrous yttria instead of Y₂O₃. It was found that the yttrium oxyapatite is unstable and attempts to prepare this silicate by a solid-state reaction always results in the formation of a mixture containing Y₂SiO₅, Y₂Si₂O₇, and yttrium oxyapatite.