Mandrel is an important tool for thermal deformation of the seamless steel tube rolling unit. It requires high heat resistance and toughness due to its application in the harsh environment. H13 steel is commonly used as mandrel materials with excellent comprehensive performance. It is reported that addition of carbide-forming elements, such as Nb, Ti, or Zr, especially the Nb element, can break the dendritic microstructure and refine the cast structure of H13 steel. In addition, Nb can act as a strong carbide-forming element to favor the formation of MC carbide. This stable carbide has low solubility and does not dissolve in austenite even at high temperature, and hence fines austenite grain by pinning effect of carbide on grain boundary. As the stable NbC has stronger ability to improve the fatigue resistance and abrasion resistance than Mo₆C and VC, the mandrel steel can be produced by the method of Nb addition. It has been reported that the addition of Nb in H13 can successfully increase heat resistance. Nb element dissolves into the matrix after quenching and tempering, and precipitates in the form of NbC after heat preservation for a long time, and eventually improves the resistance of material to temper softening. However, it has not been widely applied in the production because the primary carbides of NbC can seriously deteriorate toughness of steel. The purpose of the work is to analyze the effect of addition of 0.06%Nb (mass fraction) on segregation, primary carbides and toughness of large size H13 mandrel steel. The different segregation, primary carbides, structure between large size H13 and H13-Nb mandrel were investigated by employing methods of OM, SEM, EDS and EBSD, and the mechanical properties including the hardness and impact toughness were measured at room temperature. The results show that addition of 0.06%Nb aggravates segregation compared with H13. Nb increases the precipitation temperature of MC-primary carbides, and changes the type of MC-primary carbides from mainly VC to mainly (Nb, V)C which easily induces gravitational segregation of H13-Nb. The severe segregation leads to unfavorable structure of the large and nonhomogeneous effective grain size (EGS) of annealed H13-Nb, and the primary carbides do not decrease or change significantly after quenching and tempering. In the impact test, the zone of the chain-shaped carbides gathering is prone to cracking and generates horizontal stripes, resulting in low toughness.