A new fabrication strategy using thermal source of DC arc‐discharge plasma is developed in the synthesis of nanodiamond particles. In the fabrication process, a solid mixture of Ni/Si/graphite coarse powders is used as the raw materials, meanwhile the gaseous mixture of H₂/Ar served as the preparation atmosphere. The well‐established arc‐discharge plasma is significant in operation under a low gaseous pressure and an acceptable energy condition, and would be potential to become a novel process for synthesis of nanodiamond particles in large scale. The initially formed SiC clusters and the solid solution of Ni(C) particles are crucial in serving as crystal nuclei of diamond and an in situ carbon source which can gradually be released with cooling, respectively. Structural characterizations by high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), Raman spectra, and X‐ray diffraction (XRD) confirm the existence nanodiamond particles, they are typically spherical with 5–20 nm in size. The formation of such nanodiamond particles by the arc‐discharge thermal source is discussed in detail on the knowledge of binary phase diagram, constitution of raw materials, and nucleation/growth processes under the experimental conditions.