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The WC involving the nano metal cobalt domains has been prepared by a gas-solid reaction between the cobalt nonequilibrium supersaturated-tungsten powder and carbon mono-oxide gas (CO). The metal Co domains formed in the WC were found to be spherical with a diameter of approximately 60 nm by observing the microstructure with a high-resolution transmission electron microscope (HRTEM). The 88.36 mass% WC – 11.64 mass% Co hard metal was prepared by liquid-phase sintering at 1623 K from the powder compact which was mixture of the WC involving the Co domains and the cobalt powder as binder phase. A nano-scale fine structure composed of the WC with average diameter, dAv, of approximately 500 nm was obtained. Its Vickers hardness was found to be Hv 1663 ± 43, being nearly equal to the 95.5 mass% WC–4.5 mass% Co with 1–2 μm WC grains. That is, reduction of 7 mass% WC was accomplished. The WC involving the Co domains is expected to be used as raw materials to form the nano-scale microstructure in the hard metals, resulting in reduction in W components.