Abstract: 0Cr18Ni9 stainless steel with different Co contents was smelted by vacuum induction furnace. The effect of existing form of Co on lattice constant of phases and phase′s composition was studied by combining with microsture, phase composition and electron probe analysis. The results show that Co in 0Cr18Ni9 stainless steel mainly existed in the austenite and ferrite in the form of solid solution, the content of Co in the former was more than in the latter. Co could increase the lattice constant of austenite and δ ferrite. The number of ferrite reduced with increase of Co content in stainless steel. The lattice constant of austenite increased with decrease of the number of ferrite.