Abstract: The nanocrystalline Bi₂O₃-Y₂O₃ solid electrolyte material was synthesized by pressureless reactive sintering process with Bi₂O₃ and Y₂O₃ nano mixed powder as raw materials, which was prepared by a chemical coprecipitation process. The study on the behavior of nano δ-Bi₂O₃ formation and its grain growth showed that the solid solution reaction of Y₂O₃ and β-Bi₂O₃ to form δ-Bi₂O₃ occurs mainly in the initial stage of sintering process, and nano δ-Bi₂O₃ crystal grains grow approximately following the rule of paracurve ((D-D₀)²=K·t) during sintering process. After sintered at 600℃ for 2 h, the samples could reach above 96% in relative density and have dense microstructure with few remaining pores, the δ-Bi₂O₃ grains are less than 100 nm in size.