Zintl compound n-type Mg-3(Sb, Bi) 2 was recently found to exhibit excellent thermoelectric figure of merit zT (similar to 1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 mu m to 7.8 mu m, the Hall mobility below 500K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (Delta T = 420 K), which is quite high for a polycrystalline pristine material. Published by AIP Publishing.