Abstract: FeCoCrNiN_0.07 high-entropy alloy cold-rolled sheets prepared by powder metallurgy were annealed at different temperatures to produce ultrafine-grained and coarse-grained sheets with average grain sizes of 0.83 μm and 10.51 μm,respectively. The tensile mechanical behavior at room temperature and microstructural characteristics of high-entropy alloys with different grain sizes were studied by X-ray diffraction（XRD）, scanning electron microscope（SEM）, electron backscatter diffraction（EBSD） and transmission electron microscopy（TEM）. The results show that the tensile strength at room temperature of ultrafine-grained alloy is higher than that of coarse-grained alloy, and the elongation of ultrafine-grained alloy is slightly lower than that of coarse-grained alloy. Alloys with different grain sizes show different work hardening rate characteristics,and the work hardening rate of coarse-grained alloy is higher than that of ultrafine-grained alloy. Nano-scale deformation twins are formed in coarse-grained alloy during deformation, but no deformation twins are found in ultrafine-grained alloy.Grain refinement inhibits the formation of deformation twins in FeCoCrNiN_0.07 high-entropy alloy during tensile process.