Lead-free Na0.5K0.5NbO3 (NKN) piezoelectric ceramics were fairly well densified at a relatively low temperature under atmospheric conditions. A relative density of 96%-99% can be achieved by either using high-energy attrition milling or adding 1 mol% oxide additives. It is suggested that ultra-fine starting powders by active milling or oxygen vacancies and even liquid phases from B-site oxide additives mainly lead to improved sintering. Not only were dielectric properties influenced by oxide additives, such as the Curie temperature (T-c) and dielectric loss (D), but also the ferroelectricity was modified. A relatively large remanent polarization was produced, ranging from 16 mu C/cm(2) for pure NKN to 23 mu C/cm(2) for ZnO-added NKN samples. The following dielectric and piezoelectric properties were obtained: relative permittivity epsilon(T)(33)/epsilon(0)=570-650, planar mode electromechanical coupling factor, k(p)=32%-44%, and piezoelectric strain constant, d(33)=92-117 pC/N.