Ultrafine lead pyroniobate Pb 2 Nb 2 O 7 particles were successfully prepared via hydrothermal processing. When the lead nitrate and niobium hydrogenoxalate solutions were utilized as the starting materials, the PbC 2 O 4 precipitates formed hindered the synthesis of Pb 2 Nb 2 O 7 . On the other hand, adding ammonia to the lead nitrate solution suppressed the formation of PbC 2 O 4 while the formation rate of Pb 2 Nb 2 O 7 was significantly accelerated. Monophasic Pb 2 Nb 2 O 7 was obtained at temperatures as low as 150°C. The particle size of hydrothermally synthesized powders, compared to that of powders produced by the traditional solid state reaction, was profoundly reduced to the nanometer order (around 10-20 nm).