Nanocrystalline chromium boride (CrB) powders were successfully synthesized at 600 degrees C through a solid-state reaction. The synthesis was carried out in an autoclave by using CrCl3, Mg and MgB2 as the reactants. The X-ray powder diffraction pattern indicates the formation of orthorhombic CrB. Transmission electron microscopy image shows that typical CrB crystallites are composed of floss-like particles with less than 10 nm in diameter and about 50-100 nm in length. Thermogravimetric analysis revealed that the oxidation resistance for nanocrystalline CrB is much lower than that of bulk CrB.