Abstract: According to the principle of phase diagram thermal equilibrium and the influence of different atmosphere carbon potential, the formation process and mechanism of decarburization layer on the surface of GCr15 steel bearing ring were put forward. The results show that normal microstructure was obtained when the carbon potential of atmosphere in furnace was equal to or greater than the carbon content of equilibrium austenite in two-phase zone of the GCr15 steel. With the decrease of carbon potential, a semi-decarburized layer first appearred on the surface of the bearing ring, and the change of the microstructure was characterized by the decrease of the number of cementite, the increase of ferrite (superimposed original ferrite) and the formation of agglomerated flake pearlite. After the carbon potential continues to decrease, a full decarburization layer+semi-decarburization layer were formed from the outside to the inside of the bearing.