On-line tool wear sensing was an important subject in the Flexible Manufacturing System. Mathematic method is applied in this paper to analyze the correlation between cutting force and tool wear, and a cutting force model was established based on the tool wear. The predicted value of cutting force was calculated through the cutting test. Predicted value by comparison with the experimental data verifies the accuracy of the cutting force model. On this basis, a new method for monitoring tool wear based on measuring cutting force was proposed.