Uniform barium molybdate nano-octahedra with a mean edge length of 50nm have been prepared in Triton X-100 water-in-oil (w/o) microemulsions at room temperature. BaMoO4 nano-octahedra present tetragonal single crystals. The size of these nano-octahedra can be tuned conveniently by changing the reagent concentrations and the molar ratio of water and Triton X-100.