The synthesis of Ni/B micro-nano composite particles was studied by diamine hydrate reducing nickel chloride in the water bath at 50°C in which nano-Ni was made to cover the surface of micron boron particles. The sample was characterized by TEM, EDS, XRD, TG-DTA and thermal conductivity instrument. The results show that this method can prepare a compact coating for Ni/B composite particles. The nano-Ni particles coated on boron particles are uniform and continuous, and the mass ratio of Ni and boron is 1:1. The average diameter of Ni crystallites is about 21.7 nm. Compared with boron powder, the high temperature of exothermic peak of Ni/B micro-nano composite particles is decreased by about 36 °C. The thermal chemistry behavior of the as-prepared composite particles is better than the superfine boron powder, and the thermal conductivity of Ni/B micro-nano composite particles is higher than that of the boron powder