A polycrystalline MgB2 sample with 4 at.% Ni powder addition has been synthesized by a solid-state reaction at 750 degrees C. Two different Ni powders, raw (40 mu m) and ball-milled (10 mu m) particles were selected to clarify the influence of the Ni particle size on the secondary phase MgNi2.5B2 formation and the critical current density J(c) of MgB2 bulks. According to the investigation of the sintering, it was found that the smaller Ni particles may significantly decrease the formation temperature of the MgNi2.5B2 ternary compound to 550 degrees C. Combined with scanning electron microscopy (SEM) analysis and magnetic measurements, it was suggested that the addition of fine Ni particles led to MgNi2.5B2 inclusion precipitating along the spiral terraces of MgB2 grains and with small particles sizes. The unique microstructure is responsible for the smaller reduction in J(c), which partly eliminates the effect of weak links compared with the coarse Ni particles. Besides, the formation mechanism of the microstructures and the relative location of MgNi2.5B2 varying from cluster at the MgB2 grain boundaries to a screw arrangement around the MgB2 grains with the decrease of the Ni grain sizes were also discussed in detail. (C) 2012 Elsevier B.V. All rights reserved.