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In this study, NaNbO 3 with average grain size of ~50 nm and KNbO 3 with average grain size of ~300 nm nanocrystals are prepared by the water-based citrate precursor sol-gel process. However, the KNbO 3 sample exhibits better photocatalytic performance than that of the NaNbO 3 sample by Rh B degradation experiment. By Rietveld refinements and piezoelectric displacement measurements, the KNbO 3 with the space group of Bmm2 is ferroelectric while the NaNbO 3 with the space group of Pbma is antiferroelectric. The polarization-modulated built-in electric fields in the ferroelectric KNbO 3 nanoparticles can efficiently enhance the separation of photo-generated charge carries and thus improve the photocatalytic activity. However, there is no internal electric field in the antiferroelectric grain because of the antiparallel spontaneous polarization in the adjacent unit cell. Therefore, KNbO 3 exhibits better oxidizing ability of organic dyes than NaNbO 3 . The ferroelectric KNbO 3 nanoparticles exhibit an optimum photocatalytic performance for a complete degradation of Rh B in 100 min under UV-Vis light irradiation with auxiliary ultrasonic excitation. This study demonstrates that the perovskite-type ferroelectric nanocrystals are potentially to design high-performance catalysts for degradation of contaminant.