Formation and structural transformations of yttrium orthoferrite crystals have been studied using X-ray diffractometry, Mössbauer spectroscopy and transmission electron microscopy combined with electron microdiffraction. Said processes have been studied under heat treatment of glycine-nitrate combustion products. There have been identified formations of three structural yttrium orthoferrite modifications – amorphized hexagonal <h ₁>-YFeO ₃ (P6 ₃cm) and nanocrystalline hexagonal h ₂-YFeO ₃ (P6 ₃/mmc), as well as nanocrystalline orthorhombic o-YFeO ₃ (Pbnm), which are selectively formed depending on available three-dimensional confinements. Based on the analysis of changes in the fluid and size composition formulation, it has been proposed mechanism for formation and transformation of YFeO ₃ nanocrystals, including growth stage of h ₂-YFeO ₃ crystals due to amorphized phase of <h ₁>-YFeO ₃ up to critical size of about 15 nm and their subsequent transformation into orthorhombic form o-YFeO ₃.