MoO3 nanofibers with 50~100 nm diameter and larger than 20 μm length were prepared at 170 ℃ constant temperature for 40 h using (NH4)6Mo7O24·4H2O and nitric acid as starting materials through hydrothermal process. Then the hydrogen reduction tests of the prepared MoO3 nanofiber were carried out at different temperatures within 450~650 ℃. The samples prepared by hydrogen reduction tests were characterized by XRD and FSEM. The results indicated that MoO3 nanofibers began to change into MoO2 at 450 ℃, and could be completely reduced to Mo powder with 0.1~2 μm particle size at 600 ℃. During the reduction process, the original fiber-like morphology of most MoO3 nanofibers was not well-retained; but in some cases, the fiber-like morphology of a small part of single MoO3 nanofibers in free space could be retained, and changed into needle-like Mo particles.