The eutectic CoFeNi_2V_(0.5)Nb_(0.75) high entropy alloys(HEAs) were heated at 500, 600, 700, 800 and 1000 ℃,respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi_2V_(0.5)Nb_(0.75) HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe_2Nb-type Laves phase. The NbNi_4-type intermetallic phase appeared when the heat-treated temperature was higher than 600 ℃. With increasing quenching temperature, the volume fraction of the NbNi_4-type intermetallic phase increased while that of the eutectic regions decreased.The sample quenched at 800 ℃ showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as high as 2586.76 MPa, 2075.18 MPa and 16.73%,respectively. Moreover, the eutectic CoFeNi_2V_(0.5)Nb_(0.75) HEAs exhibited apparent age hardening, especially quenched at 700 ℃, the hardness reached up to the maximum value of HV 727.52. The eutectic CoFeNi_2V_ (0.5) Nb_ (0.75) high entropy alloys (HEAs) were heated at 500, 600, 700, 800 and 1000 ℃, respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi_2V_ (0.5) Nb_ (0.75) HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe_2Nb-type Laves phase. The NbNi_4-type intermetallic phase occurred when the heat-treated temperature was higher than 600 ° C. With increasing quenching temperature, the volume fraction of the NbNi_4-type intermetallic phase increased while that of the eutectic regions decreased.The sample quenched at 800 ℃ showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as hi gh as 2586.76 MPa, 2075.18 MPa and 16.73%, respectively. respectively, the eutectic CoFeNi_2V_ (0.5) Nb_ (0.75) HEAs exhibit apparent aging hardening, especially quenched at 700 ° C, and the hardness reaches up to the maximum value of HV 727.52.