In this study, CoCrFeNiTax (x value in molar ratio, x = 0, 0.1, 02, 0.3, 0.4, 0.5, and 0.75) high entropy alloys were prepared to investigate the alloying effects of Ta on the microstructures and mechanical properties. With the addition of Ta element, the microstructure changed from an initial single FCC solid solution (x = 0) to a hypoeutectic microstructure (x = 0.1-03), then to a fully eutectic microstructure (x = 0.4) with a mixture of FCC and Co2Ta-type Laves phases, and finally to a hypereutectic microstructure (x = 0.5 0.75). The Ta-free (CoCrFeNi) alloy showed high ductility but low strength. With the addition of Ta, the yield strength and hardness of the CoCrFeNiTax alloys increased but decreased in ductility. Therein, the fully eutectic CoCrFeNiTa0.4 alloy displayed a high fracture strength of 2293 MPa with a compression plasticity of 22.6%. The regular/uniform ultrafine lamellae structure was the primary reason for the remarkable properties of the CoCrFeNiTa0.4 eutectic high entropy alloys. (C) 2017 Elsevier B.V. All rights reserved.