Stress concentration theory was introduced into study of grinding, and the stress concentration phenomenon in grinding process was analyzed. A single-grit model of grinding force was established based on stress concentration theory. The influence mechanism of grit-wear-area rate on cutting force was clarified with the model. Functions between grinding force and grit-wear-area rate was established. The functions showed the quadratic function relationship between grinding force and grit-wear-area rate, and the increase of the cumulative residual-grinding-thickness results in the rapid increase of normal grinding force. Furthermore, by taking the feature of the grinding wheel in use into consideration, functions of grinding force for multi-grit grinding wheel was established. The validity of the model was verified by the experimental data.