The chemical doping and sintering atmosphere have great influence on the defect structure and piezoelectric properties of (Na,K)NbO 3 (NKN)-based ceramics. In this work, p/n-type NKN-based ceramics are designed by introducing different CuO doping contents. The defect configuration is analyzed by Rietveld refinement, Hall-effect measurement and impedance spectroscopy. The undoped and 3 mol% CuO-doped ceramics exhibit an n-type conduction mechanism. When n-type ceramics are sintered in air, the electron concentration will decrease, resulting in the better piezoelectric and dielectric properties than that sintered in N 2 . Nevertheless, 1 mol% CuO-doped ceramics exhibit a p-type conduction mechanism. The generation of holes can be suppressed when p-type ceramics are sintered in N 2 , which is beneficial to improve their piezoelectric and dielectric properties. The experimental results suggest that the p/n-type behavior can be judged by defect analysis and the optimal sintering atmosphere of NKN-based ceramics can be predicted by analyzing the charge carrier behavior of p/n-type ceramics.