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A series of CoCrFeNb_xNi (x values in molar ratio, x = 0, 0.25, 0.45, 0.5, 0.75, 1.0 and 1.2) high entropy alloys (HEAs) was prepared to investigate the alloying effect of Nb on the microstructures and mechanical properties. The results indicate that the prepared CoCrFeNb_xNi (x > 0) HEAs consist of a simple FCC solid solution phase and a Laves phase. The microstructures of the alloys change from an initial single-phase FCC solid solution structure (x = 0) to a hypoeutectic microstructure (x = 0.25), then to a full eutectic microstructure (x = 0.45) and finally to a hypereutectic microstructure (0.5 < x < 1.2). The compressive test results show that the Nb0.45 (x = 0.45) alloy with a full eutectic microstructure possesses the highest compressive fracture strength of 2558 MPa and a fracture strain of 27.9%. The CoCrFeNi alloy exhibits an excellent compressive ductility, which can reach 50% height reduction without fracture. The …