Ternary solid solutions of (1-x)(0.8Bi(0.5)Na(0.5)TiO(3)-0.2Bi(0.5)K(0.5)TiO(3))- xNaNbO(3) (BNKT-xNN) lead-free piezoceramics were fabricated using a conventional solid-state reaction method. Pure BNKT composition exhibited an electric-field-induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature. Accompanied with the ferroelectric-to-relaxor temperature TF-R shifted down below room temperature as the substitution of NN, a compositionally induced nonergodic-to-ergodic relaxor transition was presented, which featured the pinched-shape polarization and sprout-shape strain hysteresis loops. A strain value of similar to 0.445% (under a driving field of 55kV/cm) with large normalized strain of similar to 810pm/V was obtained for the composition of BNKT-0.04NN, and the large strain was attributed to the reversible electric-field-induced transition between ergodic relaxor and ferroelectric phase.