Lead-free Ta-modified (Na 0.53+x K 0.47+x )(Nb 0.55 Ta 0.45 )O 3 ceramics (NKNT, x D ¡0.01, ¡0.005, 0, 0.005 and 0.01) were prepared by conventional solid-state reaction. The structural and electrical properties were systematically investigated for Na/K ions non-stoichiometry. The coexistence of the orthorhombic and the tetragonal phases was confirmed near room temperature. An A-site Na/K excess improved the dielectric, piezoelectric and ferroelectric properties; in particular, the ceramic with x D 0.005 showed the highest piezoelectric constant of d 33 D 340 pC/N among all the compositions studied.