New solid solution nanocrystals with fluorite-type cubic structure in the ceria (CeO2)-yttrium niobate (1/4Y(3)NbO(7)) system were directly formed at 120 degrees C-240 degrees C from the precursor solution mixtures of (NH4)Ce(NO3)(6), YCl(3)6H(2)O, and NbCl5 under mild hydrothermal conditions in the presence of aqueous ammonia. The hydrothermal formation of cubic solid solution nanocrystals in the wide composition range of CeO2 (mol%) = 10-100 in the CeO2-1/4Y(3)NbO(7) system was effectively achieved via the assistance of the presence of CeO2 component more than 10mol% as a promoter with the same fluorite-type structure. The optical band gap of the solid solutions gradually decreased with increased CeO2 component. The high phase stability of the solid solutions in the CeO2-1/4Y(3)NbO(7) system was confirmed, i.e., the single cubic phase of the solid solutions was maintained after heat treatment at 600 degrees C-1500 degrees C for 1h in air. The presence of Y3NbO7 as an inhibitor and the substitutional incorporation of Y3NbO7 into the lattice, CeO2 effectively controlled the crystallite growth of CeO2, and nano-sized cubic solid solutions with high specific surface areas were maintained after heat treatment up to 800 degrees C-1000 degrees C for 1h air.