To provide a process for producing ceria-zirconia solid solution crystal fine particles having high crystallinity, being excellent in uniformity of composition and particle size, having a small particle size and a high specific surface area and being excellent in heat resistance, and such solid solution crystal fine particles.A process comprises a step of obtaining a melt containing, as represented by mol % based on oxides, from 5 to 50% of (ZrO2+CeO2), from 10 to 60% of RO (wherein R is at least one member selected from the group consisting of Mg, Ca, Sr and Ba) and from 25 to 70% of B2O3, a step of quenching the melt to obtain an amorphous material, a step of heating the amorphous material at a temperature of from 550 to 1000° C. to precipitate a ceria-zirconia solid solution crystal in the amorphous material, and a step of separating components other than the ceria-zirconia solid solution crystal from the obtained precipitates to obtain the ceria-zirconia solid solution crystal fine particles, in this order.