In this study, the non-stoichiometric composition [(Na 0.5 K 0.5 ) 0.97 (Nb 0.96 Sb 0.04 )]O 3 +xmol% CuO+0.2wt% Ag 2 O (where x=0, 0.2, 0.4, 0.6, 0.8, 1.0, respectively) ceramics were fabricated as a function of the amount of CuO addition. Then, their dielectric and piezoelectric properties were investigated. With a 0.8mol% CuO content, these ceramics (abbreviated as NKNS) showed excellent values of density=4.459g/cm 3 , electromechanical coupling coefficient (k p )=0.469, mechanical quality factor (Q m )=540, dielectric constant (ɛ r )=410 and piezoelectric constant (d 33 )=70pC/N at the sintering temperature of 1080°C, respectively.