The grinding wheel in the grinding process will surely become duller and duller. To find the reason for it is beneficial to optimize grinding process. The paper draws the graph of the Passivation process of grinding wheel by carrying grinding experiment in the condition of invariable grinding force. The curve can be divided into three parts according to the Passivation rate i.e. initial dull stages, regular dullness stages and rapid dullness stages. In the initial stages, the Passivation rate is lowest and the grinding efficiency is the greatest. The curve figure changes from concave to protruding，which is greatly different from wear curve of grinding process.