Abstract: Ultrafine nickel (Ni) powder was successfully synthesized by supercritical methanol hydrothermal synthesis. The effects of reaction conditions, including precursor salt solution concentration and pH, reaction temperature, as well as the type and concentration of complexing agents, on the ultrafine Ni powder were systematically investigated. The results indicated that spherical ultrafine Ni powder with high crystallinity and an average particle size of (257.1±53.7) nm was successfully synthesized under conditions of a reaction temperature of 250 ℃, pressure of 12 MPa, reaction time of 5 min, pH of 12.82 and initial NiCl₂ solution concentration of 0.05 mol/L. The antioxidant performance tests showed that the peak oxidation temperature of the ultrafine Ni powder prepared by the supercritical methanol method was 536.7 ℃, significantly higher than that of the commercial Ni powder (456.8 ℃), demonstrating superior antioxidant properties.