Using (Bi2O3)0.75(Dy2O3)0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi–O binary system, the sintering conditions under SPS were optimized. (Bi2O3)0.75(Dy2O3)0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi2O3)0.75(Dy2O3)0.25 followed a parabolic trend, expressed as D2 − D02 = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol− 1. Dense (Bi2O3)0.75(Dy2O3)0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the Î' phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi2O3.