Iridium is expected as the most promising base metal for future ultra-high temperature structural materials. With large atomic size misfit to Ir, Hf and Zr were found to be the most effective solid-solution and precipitate hardening elements. Multi-component alloying of Ir by Hf and Zr is employed in this work to promote the high-temperature mechanical properties of Ir-based alloys. The Ir-15Hf binary alloy was used as a base material and Ir was further replaced by 1, 5, 10, and 15 mol % Zr. The results showed that with an increasing Zr content the microstructure of the Ir-15Hf-(1-15) Zr alloys changed from two-phase fcc/L12 to L12/L12 structure. From room to high temperatures, considerable hardening took place when the microstructure contained significant amount of saturated fee phase, while the L12 dominating or two-phase L12/L12 microstructure showed lower hardening efficiency. Even at 1800°C, the Ir-15Hf-1Zr alloy containing significant amount of saturated fcc-phase had the yield 0.2% strength of as high as 340MPa. The intergranular fracture could govern the failure of the Ir-Hf-Zr ternary alloys. Finally, a principle for the design of the Ir-based alloy with high strength at elevated temperatures based upon the composition and fracture mode was discussed and proposed.