We report a new method to synthesize cobalt (Co) nanoparticles using supercritical hydrothermal synthesis. An important aspect of supercritical hydrothermal synthesis of nano particles is the ability to create a homogeneous reducing atmosphere by introducing H₂ gas. Co aqueous solution with formic acid was pressurized to 30 MPa at room temperature and rapidly heated to 400°C by mixing with supercritical water and then fed into a tubular reactor. The size of obtained Co nanoparticles was about 10nm and its particle size distribution was very narrow. The coercivity force at room temperature of the nanopartricles showed about 90 Oe. In this study, another important aspect of supercritical hydrothermal synthesis is reported; that is, in-situ surface modification of Co nanoparticles with carboxylic acids during crystallization.