A new branched morphology of metallic niobium powder particles was produced by calciothermic reduction of niobium oxides in molten CaCl2. The influence of some reaction parameters on the powder particle morphology was studied, and the formation mechanism was investigated. It was found that the formation of intermediate complex niobium oxides (CaxNb2Oy) was crucial in the formation of niobium powder particles with the branched morphology. Based on this finding, the formation mechanism of the branched morphology in four stages was proposed. These Nb powder particles with microscopic branches can prevent particle sintering during the fabrication process of electric capacitors, allowing the latter to exhibit a higher capacitance.