The size-controlled preparation of Mo powders is always a challenge and important task in the molybdenum metallurgy. In the current study, Mo powders with controllable sizes are synthesized by hydrogen reduction of MoO2 powders with the assistance of Mo nuclei in the range of 900-1100 degrees C. The influences of the particle sizes of Mo nuclei, the additive amount as well as reaction temperature on the morphology and particle sizes of the final products are studied. For the hydrogen reduction of MoO2 without any additive, the obtained Mo powders always have large particle sizes. However, the addition of small amounts of nuclei in MoO2 can help Mo nucleate dispersedly, and the growth of Mo could be also controlled by adjusting the sizes of added nuclei, amount of addition and the reaction temperature. With the addition of Mo nuclei, the different sizes of Mo powders with the good dispersity can be prepared. As adding commercial Mo powders with the particle size of about 2.03 mu m, the micron-sized Mo powders ranged from 2.11 mu m to 3.25 mu m could be prepared. While for the case of adding ultrafine Mo nuclei of about 170 nm, Mo powders from 0.28 mu m to 0.88 mu m can be obtained. Moreover, the more the amounts of nuclei added and the lower the reaction temperature (in the range of 900-1100 degrees C) is, the smaller the particle size of the prepared Mo powder will be. The current method is a facile and feasible method, and is potential to be used for industrial production of Mo powder with controllable particle sizes. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.