Piezoelectric responses of Bi0.5(Na0.82K0.18)0.5TiO3 (NKBT) ceramic films with thickness of 10-100 μm have been studied in a wide range of ac electric field and frequency. By Rayleigh law fit, the typical irreversible Rayleigh coefficient, α, for piezoelectric response is 39.69±0.41×10−18 m2V−2, which is much larger than that of the piezoelectric thin films, indicating significantly enhanced non-180° domain wall motions in NKBT thick films. The logarithmic frequency dependence of the piezoelectric response is associated with domain wall pinning in NKBT thick films, and the frequency dispersion increases with increasing ac field amplitude, which is attributed to the relaxation of non-180° domains.