Steel alloy of 1.2379, including tool steels group high chromium-high carbon is that extremely high wear resistance with high dimensional stability, this type of steel suitable for making molds. This type of steel during the solidification of chromium and carbon alloy element separation, network of chromium carbide phase in austenite grain boundry formed that the properties damages the cast steel. Network of carbide in the high temperature heat and the long time can not dissolve and warm forging operations to break the eutectic chromium carbides and improve mechanical properties can be used but at large ingots with network chromium carbides during forging cause deep cracks and lead to collapse. In present study for chemical modifications of network of eutectic carbides network and thus improve the toughness of alloy properties, agents modifier Mg-RE-B and Ce-La and niobium element separately were added to molten alloy after casting and annealing heat treatment, on samples microstructure study and mechanical test was performed. The results showed that adding both modifier operating in steel, chromium carbides morphology due to change from continuous to discontinuous mode network and also reduce the size of chromium carbides and increasing the dissolution of carbides during heat treatment. Niobium to cause formation of niobium carbides and chromium carbides reduce the other hand, change the morphology of chromium carbides, shrink its and shrink austenite dendrities and uniform distribution in the alloy was cast in the state. Add modifier factors Mg-RE-B and Ce-La alloy to melt the hardness without loss, failure force at room temperature and strength ultimate tensile alloy at a temperature of 600 ° C and percentage elongation increased. Add the niobium 1.5 percent of the alloy increased stiffness and increased failure force at room temperature, the ultimate tensile strength of alloys at temperatures 600 ° C did not change much but the percentage elongation at this temperature for more than three times the increase . Add 1.5 percent niobium to the alloy of optimum hardness, toughness and high tensile strength, among other samples originated brings.