The elastic constants,ideal tensile strength (ITS),stacking fault energy (SFE),lattice constant and mag-netic moment of FeMnCoCrNi high entropy alloys with varying Co and Cr contents at 0 and 300 K were systematically investigated by first-principle calculations,For the alloys with Co substitution for Ni,at both temperatures the elastic stability of the face-centered cubic (fcc) phase,bulk elastic modulus (B),Young's modulus (E),shear modulus (G) and ITS increase monotonically with increasing Co content.How-ever,the Cauchy pressure (Cp),Pugh ratio (B/G),Poisson ratio (v),Zener anisotropy ratio (Az) and elastic anisotropy ratio (AvR) decrease monotonically.The SFE also decreases with the increase of Co,resulting in the change of plastic deformation mechanism from dislocation slip to mechanical twinning,and then to hcp-martensitic transformation.This elucidates the underlying mechanism of the effect of Co addition on the strength and micromechanical behavior of FeMnCoCrNi alloys.Compared with Co,the Cr substitution for Ni leads to the more complex change of elastic constants and ITS.The increase of Cr shows the similar effect on SFE and deformation mechanism as that of Co.The variation of valence electron concentration and magnetism affect the SFE.The increase of either Co or Cr leads to the reduced magnetic moments of Fe and Mn.This could be responsible for the monotonic decrease of both lattice constant and SFE as the Co content increases.However,for the Cr addition case,multiple factors may affect the evolution of lattice constant and SFE.These findings shed light on the deformation mechanism of the alloys with different compositions.