Site specific substitution of K-ion at Na-site (i.e., Na0.5−xKxBi0.5TiO3-NKBT) by varying its concentration from x = 0–10 mol% decreased the grain size significantly from 14 μm (x = 0) to 1.1 μm (x = 10). The substituted K-ion acts as a grain growth inhibitor to attain fine grained ceramics. Coercive field (EC) showed a strong grain size dependence and observed a maximum softening of 43% in EC i.e., 63 kV/cm (x = 0) to 36 kV/cm (x = 10). Vickers hardness also displayed strong grain size dependence and attained a maximum of 10.2 GPa for NKBT-10 ceramics, which is double than parent NBT and other commercial lead based piezo-ceramics such as PZT and PLZT. © 2017 Elsevier B.V.