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Ni nanoparticles were successfully introduced for preparing polycrystalline MgB 2 samples by a novel reduction method. According to transmission electron microscopy, the obtained Ni nanoparticles with average grain size of 5 nm are distributed in the B matrix without agglomeration. The missing shift in X-ray diffraction peaks indicates that Ni nanoparticles could not substitute Mg sites in the lattice, but rather form the ternary compound MgNi2.5B2 above 600°C. The low-melting eutectic liquid formed by Mg-Ni at 506°C is responsible for the formation of plate-like MgB2 grains and the fast fabrication of MgB2 at low temperature. The sample doped with nano-Ni has better grain connectivity and the critical current density (Jc) with respect to the commercial Ni-doped sample, but there is no improvement compare with the pure MgB2 sample. A sintering model was accounted for understanding the reaction between Mg and B with the assistance of Ni nanoparticles.