In order to on-line monitor tool wear, an on-line monitor tool wear system was built. The tool wear can only be meseaured indirectly in work. Hence, the motor current and the tool wear is linear dependence. First, the relation model between motor current and cutting parameter of CNC was built, and then on-line monitor the current based on uC/COS, which can be monitor tool wear indirectly was done. The experiment results showed that the model is accuracy, and sensitivity Analysis of parameter was carried out.