Nanostructured iron and its alloys are industrially important materials. For example M50 steel (4.0% Cr, 4.5% Mo, 1.0% V, 0.8% C and balance Fe) due to its resistance to tempering, wear and rolling contact fatigue has extensive applications in the aircraft industry as main shaft-bearings in gas turbine engines [1, 2]. On the other hand, ultrafine iron dispersions have applications in ferrosuids, magnetic memory systems and catalysis [3, 4]. Conventional compacted M50 steel contains large micrometer sized carbide particle [5-8] dispersed in a body-centred tetragonal martensite phase and the former can act as fatigue crack initiation sites. In contrast, the nanostructured steels which would not have the large carbide particles would therefore be expected to exhibit superior mechanical properties. Several physical and chemical methods exist for the synthesis of nano-structured powders [9-11] such as metal evaporation [12], spray pyrolysis [13, 14], laser ablation [15,16], decomposition of metalorganics [17], chemical reduction [18, 19] and solgel processing [20, 21]. Decomposition of transition metal carbonyls using ultrasound has earlier been used to synthesize transition metal nanopowders [22,23] and some groups have reported the preparation of ultrafine iron dispersions in functional polymers by the thermal decomposition of Fe(CO)5 [24, 25], however, there have been no reports in the literature on the compaction of sonochemically produced nanometric iron powders and their characterization. The chemical effects of ultrasound are numerous and it can dramatically improve the rates and yields of stoichiometric and catalytic reactions [26, 27]. This property of ultrasound is utilized in this work to synthesize, characterize and subsequently consolidate nanometer-sized iron particles and nanophase M50 steel powders by the decomposition of organometallic precursors. The iron and M50 steel powders were synthesized in a one-step reaction by the sonochemical decomposition of the organometallic precursors in a high boiling point solvent, namely decalin. The powders