Abstract: This paper deals with the effects of cobalt on the microstructures of Fe-Cr-Ni-Co base superalloy Refractoloy26, cobalt is replaced by nickel to produce versions of alloy containing 0, 7, 14, and standard 20 pct.cobaltr espectively.The microstructures of these alloys arc examined by optical and electron microscope,X-ray diffractionand chemical phase analysis technique. As cobalt decreases, the weight fraction ofγ'phase as well as the average diameter of γ' particles increase,while theγ' lattice mismatch decreases. It is found that removing cobalt destabilizes MC carbide in the alloys so that t more grain boundaries M₂₃C₆ and M₆ C carbide form during ageing treatment,and the amount of carbides increase.The results of X-ray diffr-action analysis illustrate that the M₂₃C₆ carbide disappears but μ-phase forms in the alloys with 14 and 20 pct.cobalt.Optical quantative analysis re-suits show that as cobalt content increases, the amount of twin increases, the morpholoy of twin boundary is changed.It is directly depended on the effect of cobalt morphloy on the stacking fault energy of matrices.