x(NaF−0.5Nb2O5)−(1 − x)[(Na0.5K0.5)(Nb0.8Ta0.2)O3] (100xNN−NKNT) piezoelectric ceramics were fabricated to high densities above 97% when sintering at temperatures ~1200°C. Compared with pure (Na,K)NbO3 (NKN), dielectric constants of the NaF–Nb2O5 flux-doped NKNT ceramics were increased whereas dielectric losses remained low. High field polarization switching showed very square hysteresis loops, but the coercive fields were decreased through a “softening” doping effect induced by the flux. A Rayleigh analysis inferred that the extrinsic contribution from the domain wall dynamics was increased with the flux addition. Collectively, all the experimental observations suggested that NaF–Nb2O5 was, in part, soluble in the structure and highly suitable for obtaining a stable NKN system with improved piezoelectric performance.