It was tried to find the optimal condition to prepare the metallic niobium powder with minimal oxygen content by atmospheric magnesium-gas reduction of niobium pentoxide (Nb₂O₅) powder at 1073–1223 K, which are industrially moderate and low temperature ranges until maximally 80 h inside the chamber held under the argon circumstances of 110 kPa. Magnesium oxide of the by-product of the reduction, was dissolved and removed fully by dissolving in a 10% aqueous hydrochloric acid solution. The particle size of the niobium powder reduced for 20 h was slightly increased within the range of 200∼600 nm according to increase of reduction temperatures. And such fine particles were further coarsened to near 1 µm by increase of reduction times until 80 h at 1173 K, which is thought to be the most suitable for magnesium-gas reduction to be applied in industry. The reduction time satisfied for a maximal reduction effect was found to be 60 hours as the oxygen content was then minimally saturated to about 0.42 wt.%. Furthermore, the hydrogen contamination due to acid leaching of 0.28 wt.% was fully removed by dehydrogenation, which was a heat treatment performed under vacuum at 827 K for 2 h; this resulted in the formation of metallic niobium powder.