Nanocomposite ultrafine powders of thorium oxide and tungsten molybdenum alloy were prepared by arcing between a pure molybdenum cathode and a thoriated tungsten anode under an argon-hydrogen atmosphere. The microstructure, morphology and chemical ana lyses of the ultrafine powders were examined by X-ray diffraction, transmission electron microscopy and associated techniques. The powders are composed of thorium oxide single-crystalline nano-grains decorated by tungsten molybdenum alloy particles which are crystallized in a metastable A15 structure. Two morphologies were observed : octahedra and needle-like particles. Every octahedral particle bigger than 15 nm had on each corner a metallic grain generally less than 8 nm in size. The needles had a length below 600 nm and their width to length ratio ranged from 1/3-1/15. They all presented on one of their ends a metallic particle which was comparable in size to the needle width. Other smaller particles (< 10 nm) may be encountered along the edges. The metallic particles were found to be epitaxial on the thorium oxide grains according to the relation : (100)[011](ThO2)parallel to(100)[011](A15). The extensive study of the microstructural and morphological properties of the powders leads us to suggest a growth mechanism of the nanocomposite particles.