CuO-added 0.96(Na0.5K0.5)(Nb1-xSbx)O-3-0.04SrTiO(3) ceramics sintered at the low temperature of 960 degrees C for 10hours showed dense microstructures and high relative densities. The specimens with 0.0 x0.04 had orthorhombic-tetragonal polymorphic phase boundary (PPB) structure. Tetragonal-pseudocubic PPB structure was observed in specimens with 0.05 x0.07, while the specimen with x=0.08 has a pseudocubic structure. The structural variation in the specimens is explained by the decreases in the orthorhombic-tetragonal transition temperature and Curie temperature with the addition of Sb5+ ions. The specimens with 0.05 x0.07, which have tetragonal-pseudocubic PPB structure, had large electric field-induced strains of 0.14%-0.016%. Moreover, these specimens also showed increased d(33) values between 280 pC/N and 358 pC/N. In particular, the specimen with x=0.055 showed particularly enhanced piezoelectric properties: d(33) of 358 pC/N, k(p) of 0.45, and the electric field-induced strain of 0.16% at 4.5kV/mm.