Abstract: A refractory containing graphite is commonly used in the metallurgical industry in locations subject to severe thermal shock because of the high thermal conductivity and good thermal shock resistance of graphite. However, a refractory that uses phenolic resin as the carbon precursor is brittle, and to improve its strength and toughness, Ni(NO₃)₂ is added to the resin to catalyze the in-situ formation of carbon nanofibers/nanotubes. The microstructure and mechanical properties of the Ni(NO₃)₂-modified phenolic resin carbons were characterized by XRD, SEM, TEM and mechanical tests. Results indicate that carbon nanofibers/nanotubes (2% by volume) were formed within the pyrocarbons as a result of the nickel catalyst and these are interconnected to form a network structure. The nanocarbon fibers/tubes significantly improve the bend strength, elastic modulus, tensile strength and fracture toughness of the pyrocarbons and their fracture energies are increased accordingly.