High-speed machining is investigated in a new viewpoint, thermodynamic theory. Many phenomena in cutting, such as heat conduction, chemical reaction, diffusion and so on, can be studied with thermodynamic theory. In this paper, entropy generation and entropy flow in cutting system are revealed as well as the interaction of sub processes. As a case study, the oxidation of cemented carbide tool was studied in thermodynamic theory and experiments. The results show that thermodynamic theory is effective.