Fabrication of full-densified Mo metal with fine grain size and uniform microstructure is highly desired to improve its mechanical property. In this work, microwave-field-assisted thermal sintering was utilized to promote the fast densification of Mo nanocrystals, and the compact with high relative density of 99.8 % and fine grain size of 2.09 μm was obtained at 1450 °C for 0.5 h with heating rate of 20 °C/min, which is much superior to the compact with density of 99.13 % and grain size of 7 μm fabricated by traditional thermal sintering at 1500°C for 1 h. Further theoretical and experimental investigation reveals that effective volume heating transfer with low activation energy provides strong driven-force for densification of nanosized Mo powder at low temperature.