Five refractory high-entropy alloys with nominal composition of NbMo0.5HfxTiZrCrAl (x=0, 0.25, 0.5, 0.75, 1, at.%) were prepared by vacuum non-consumable arc smelting. The effects of Hf addition on the microstructure and mechanical properties of the alloys were studied. The results show that the microstructures of the alloys with and without Hf are composed of two BCC phases (namely BCC1 and BCC2) and Laves phase. In the alloy without adding Hf, the BCC1 phase is mainly rich in Zr, Cr and Al elements, and the BCC2 phase is mainly rich in Nb, Ti and Mo elements. In the Hf-added alloys, the BCC1 phase is mainly rich in Hf, Zr, Cr, Mo and Al elements, and the BCC2 phase is mainly rich in Nb and Ti elements. With the increase of Hf content, the amount of BCC1 phase gradually increases, and the lattice constants of BCC1 and BCC2 phases both increase. In addition, with the increase of Hf content, the microhardness and brittleness of the alloy gradually increase, and the compressive strength of the alloy at 1200 ℃ gradually decreases.