Ultrafine lead pyroniobate Pb2Nb2O7 particles were successfully prepared via hydrothermal processing. When the lead nitrate and niobium hydrogenoxalate solutions were utilized as the starting materials, the PbC2O4 precipitates formed hindered the synthesis of Pb2Nb2O7. On the other hand, adding ammonia to the lead nitrate solution suppressed the formation of PbC2O4 while the formation rate of Pb2Nb2O7 was significantly accelerated. Monophasic Pb2Nb2O7 was obtained at temperatures as low as 150 degrees 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).