Controlled synthesis of sodium potassium niobate (NaxK 1-xNbO3: NKN) fine particles with different K/Na ratios was successfully achieved by the two-step hydrothermal reaction at 100 °C for 24 h and 200-250 °C for 3 h using niobium pentachloride as a soluble precursor. As a result of the effect of the K/Na ratio in the starting solution, NKN particles with an orthorhombic sodium niobate were formed in the NaOH/KOH ratios from 10/8 to 7/11. When the NaOH/KOH ratio was adjusted to 10/8, cubic-shaped particles were obtained, and the mean particle size with the size distribution was 3.0 ± 1.2 μm. In contrast, by aging at 250 °C, NKN particles with orthorhombic potassium niobate crystal structure were obtained in the NaOH/KOH ratios from 5/13 to 1/17. Furthermore, monodispersed and octahedral-shaped NKN fine particles with a tetragonal crystal structure were formed as a single phase at 200 °C with the NaOH/KOH ratio of 6/12. The octahedral-shaped particles had a hierarchical built-up structure of cubic-shaped nanoparticles. The sodium and potassium ratio in the NKN crystal structures was further characterized by Rietveld analysis. All the NKN ceramics, prepared starting from the present hydrothermal method, had a highly porous structure. However, these ceramics exhibited high d33 values of ca. 100 pC/N. This result means that the octahedral-shaped NKN particles have high potential as lead-free piezoelectric materials.