High stable nickel nanoparticles (NiNPs) have been successfully synthesized from nickel chloride as the precursor through a simple one-pot solvothermal process. A systematic investigation of the reaction parameters, namely, effects of reaction temperature, pH of precursor solution, the concentration of reactants, and reaction time on the formation of NiNPs, was carried out to obtain the optimal values for the synthesis. The optimum reaction temperature, pH, NiCl2·6H2O concentration, and reaction time are 190°C, pH 9, 0.1 M, and 24 h. The characteristic peaks of NiNPs have been confirmed by the Fourier transformer infra-red and surface plasmon resonance, with the presence of –OH stretching bands at 3,593 and 603 cm−1 (interaction with Ni and NiO) and λ max 265 nm, respectively. The X-ray diffraction and transmission electron microscope demonstrated the particle size of about 24 nm (by Scherrer) and 49 nm (Image-J), respectively, with the face center cubic phase. The synthesized NiNPs showed good stability, where the degradation of NiNPs was completed at 800°C with more than 97% residue as depicted by the thermogravimetry analysis. The synthesized NiNPs can be used as fillers to enhance the thermal, mechanical, and electrical properties of polymeric materials.