It has been reported that the rhombohedral-orthorhombic low-temperature polymorphic phase transition in (Na,K)NbO(3) can be tuned close to room temperature by substituting Sb for Nb, such that enhanced piezoelectric properties are induced based on the theory of two-phase coexistence. A diagram of phase structures changing with Sb content and temperature has been generalized for (Na,K)(Nb,Sb)O(3) (NKNS) compositions. The thermal stability of piezoelectric properties of NKNS ceramics was evaluated considering the existence of successive phase transition above room temperature. The microstructure, dielectric, and ferroelectric properties of NKNS ceramics were discussed from a crystallographic point of view.