The as-cast CoCrFeNiMnCx (x = 0, 0.05, 0.1,1.5 and 2.0) alloys were prepared in a vacuum arc furnace. The microstructure and phase constituents were characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy, and the tensile properties were tested as well. The results show that the CoCrFeNiMn and CoCrFeNiMnC0.05 alloys have a single phase FCC structure. When the carbon concentration is beyond 0.1 (x = 0.1, 1.5 and 2.0), the M7C3 carbide generates in the inter-dendritic regions and at the grain boundaries. Minor C addition can trigger a transition from dislocation glide dominated plasticity to a mixed deformation mode consisting of dislocation glide and twining, which increases the strength and ductility of the CoCrFeNiMnC0.05 alloy compared with the CoCrFeNiMn alloy. The CoCrFeNiMnC0.05 and CoCrFeNiMnC0.1 alloys have a typical lamellar pattern on the fracture surface due to the formation of columnar grains and the segregation of C at the grain boundaries. The alloys become stronger with the increase of carbon concentration, but at x > 0.1, the ductility decreases and the CoCrFeNiMnC0.15 and CoCrFeNiMnC0.2 alloys present a quasi-cleavage fracture mode. (C) 2017 Elsevier B.V. All rights reserved.