The effects of Re-Nb on the hardness, tensile strength, Charpy impact energy, and fracture toughness of 3Cr2MoNiWV cast hot working die steel have been investigated. It is shown that the tensile strengths at room temperature and at 600 degrees C increased by 19% and 22% respectively while Charpy impact energy and fracture toughness increased to 109% and 70% respectively while the hardness remain unchanged by Re-Nb modification. The results showed that Re-Nb modification can refine the microstructure of the steel, increase the volume fraction of lath martensite and retained austenite in the tested steel and change the morphology of non-metallic inclusions from bar-like to a fine nodular type. The results are explained based on the modification of the microstructure due to Re-Nb modification. (c) 2006 Springer Science + Business Media, Inc.