The (Na0.52K0.44)(Nb0.9Sb0.06)O3-0.04dLiTaO3 (NKNS-LT) ceramics with various Cu2O concentration were prepared by the conventional solid state reaction method. The Cu2O content was varied in the range of 0.1~0.4 wt%. The effects of Cu on microstructure, crystallographic phase transition, and piezoelectric properties were investigated. The material with perovskite structure had a tetragonal phase (T1) when Cu2O concentration was less than 0.3 wt% and it transformed to another tetragonal phase (T2) when the Cu2O amount was greater than 0.3 wt%. The phase boundary between T1 and T2 phases appeared at around 0.3 wt% of Cu2O concentration. The piezoelectric properties were shown the maximum values at the composition of the phase boundary. The electro-mechanical coupling factor (kp) was 0.42 and the piezoelectric charge constant (d33) was 245 pC/N at the 0.3 wt% of Cu2O concentration.