High temperature strength and room temperature fracture toughness of Nb-based alloy system were investigated to explore a candidate material for gas turbine use at 1773 K as a function of chemical compositions. 0.2% proof stress at 1773 K and the fracture toughness were studied for the solid-solution hardening Nb-Mo-W alloys and the Nb₅Si₃-reinforcing Nb-Si-Mo-W-Hf alloys, and were found that the 0.2% proof stress linearly increases with increasing (Mo+1.5W) content and also increases with volume fraction of Nb₅Si₃, while the fracture toughness decreases with (MO+1.5W) content in solid-solution. The critical content for a ductile-to-brittle transition to occur is at (Mo+1.5W) content of 33.5 mol% together with molybdenum content of 15 mol%. Nb-16Si-5Mo-15W-5Hf alloy with a microstructure consisting of 50 vol% solid-solution and 50 vol% Nb₅Si₃, obtained by optimizing the alloy chemistry without a significant loss of high temperature capability and ductility, and by modifying with 5Hf-5C addition, was found to demonstrate an unprecedentedly excellent creep resistance and also a creep rupture strength exceeding the target strength of 150 MPa and above at 1773 K for 100 h.