We studied crystallization, grain growth and electric properties of La0.8Sr0.2MnO3 (LSM) ceramics which were produced using the microwave-treatment. While co-precipitated nanoparticles remain mainly amorphous, the microwave irradiated particles are crystallized into LSM and La2Mn2O7 at 550 degrees C, due to higher dielectric polarizability of La. This, in turn, decreases the amount of the second phase La2O3 in calcined powder and promotes the growth of perovskite grains during sintering at 1400 degrees C. Larger grains of LSM ceramics lower the activation energy of small polaron hopping from 0.35 eV to 0.24 eV and increases high-temperature electric conductivity. In addition, high crystallinity of LSM ceramics from the microwave-treatment suppresses a chemical reaction with ZrO2 and NiO in a temperature range of 900 - 1100 degrees C under oxidizing and reducing ambiances. These results show that LSM ceramics from the microwave-assisted reaction meet requirements for an interconnect layer for solid oxide electrolysis cells.