Grain size and piezoelectric properties were investigated in (K, Na)NbO₃ ceramics substituted with Ba for K and Na by 1 to 5 mol%, in anticipation of high piezoelectric properties. The grain size strongly depended on the A/B ratio in perovskite structure. The grain growth phenomenon was markedly similar to that of BaTiO₃ semiconducting ceramics with donor additives. Piezoelectric properties were strongly affected by grain size. MnO additive was added to suppress grain growth anomaly, dense ceramic body formation and withstanding voltage enhancement. However, a manufacturing process, in which MnCO₃ was directly mixed to other raw material powders, resulted in the formation of large pores ranging to 1 mm diameter in their ceramic bodies. The powder obtained by previously calcining the mixture of Nb₂O₅ and MnCO₃ was used as a raw material for MnO addition, in order to prevent the formation of large pores. By adding 0.5 mol% MnO, grain growth anomaly was inhibited, and a uniform and dense ceramic body was consequently obtained without piezoelectric property degradation.