Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO3 and KNbO3 have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na0.5K0.5NbO3 ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920 degrees C, the density of the Na0.5K0.5NbO3 solid solution ceramics was raised to 4.47 g/cm(3) (> 99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na0.5K0.5NbO3 ceramics with submicron grains. The crystal phase of the Na0.5K0.5NbO3 has an orthorhombic structure. The Curie temperature is 395 degrees C and the piezoelectric parameter (d(33)) of the Na0.5K0.5NbO3 ceramics reached 148 pC/N.