Nanopowders of 0.63Pb(In1/2Nb1/2)O-3-0.37PbTiO(3) were synthesized by solid state reaction using the continuous attrition milling followed by high-energy ball milling techniques in air at room temperature. After milling for 8 h nanopowders of 20-30 nm grain size are obtained. Sintering by hot pressing of PIN-37PT green pellets leads to dense ceramics with average grain size varying from 100 nm to 1 mu m. The dielectric and piezoelectric properties of PIN-37PT nanostructured ceramics with grain size bigger than about 160 nm remain roughly unchanged and comparable to those of microstructured ceramics. In addition, the stability of the permittivity and dielectric losses under high ac electric field grows when the grain size decreases. The material becomes less non-linear with decreasing grain size. This result is attractive for acoustic transducer applications.