In this study,a new Al0.9CoFeNi2 eutectic high entropy alloy(EHEA)was designed,and the microstruc-tures as well as the deformation behavior were investigated.The bulk cast Al0.9CoFeNi2 EHEA exhibited an order face-centered cubic FCC(L12)and an order body-centered cubic(B2)dual-phase lamellar eutec-tic microstructure.The volume fractions of FCC(L12)and B2 phases are measured to be 60％ and 40％,respectively.The combination of the soft and ductile FCC(L12)phase together with the hard B2 phase resulted in superior strength of 1005 MPa and ductility as high as 6.2％ in tension at room temperature.The Al0.9CoFeNi2 EHEA exhibited obvious three-stage work hardening characteristics and high work-hardening ability.The evolving dislocation substructures during uniaxial tensile deformation found that planar slip dominates in both FCC(L12)and B2 phases,and the FCC(L12)phase is easier to deform than the B2 phase.The post-deformation transmission electron microscopy revealed that the sub-structural evolution of the FCC(L12)phase is from planar dislocations to bending dislocations,high-density dislo-cations,dislocation network,and then to dislocation walls,and Taylor lattices,while the sub-structural evolution of the B2 phase is from a very small number of short dislocations to a number of planar dis-locations.Moreover,obvious ductile fracture in the FCC(L12)phase and a brittle-like fracture in the B2 phase were observed on the fracture surface of the Al0.9CoFeNi2 EHEA.The research results provide some insight into the microstructure-property relationship.