By the traditional solid-state reaction method, Bi-0.5(Na0.8K0.2)(0.5)Ti1-x (Fe0.5Nb0.5) (x) O-3 (marked as BNKT-xFN; x = 0.00, 0.01, 0.02, 0.03 and 0.04) piezoelectric ceramics were fabricated, and the effects of FN substitution on the dielectric, ferroelectric, piezoelectric and electric field-induced strain performance were investigated. The relative dielectric permittivity and loss tangent reveal that the phase transition temperature between ferroelectric and ergodic relaxor phase is reduced from 90 A degrees C to room temperature (RT), even below RT, with increasing FN content. Temperature dependence of polarization-electric field loops and strain-electric field curves exhibits that the ferroelectric order is disturbed gradually, and the ergodic relaxor phase forms with increasing FN content. A large unipolar strain of 0.42% and corresponding d (33)* (= S (max)/E (max)) of 642 pm/V for the sample with x = 0.04 are obtained under 6.5 kV/mm due to the phase transition from ergodic relaxor to ferroelectric. These results indicate that the (Fe0.5Nb0.5)(4+) complex ion-modified BNKT-based ceramics would have great potentials for lead-free electromechanical actuator applications.