High-speed cutting and hot-machining of Cr-Mo steels are carried out. It is found that the shear stress on the tool face is always lower by some amount than that on the shear plane. In high-speed cutting the shear stresses on the shear plane and tool face are also found to remain about constants respectively with increasing cutting speed, although these stresses decrease with increasing heating temperature in not-machining. The flow stresses in cutting, as well as in material tests at room and high temperature, are discussed, taking into consideration the effects of temperature and strain rate. Then it is shown that these flow stresses are expressed by a unique function of the velocity-modified temperature θm, which is defined as follows : [numerical formula] where[table] Above mentioned features of flow stress in cutting may be satisfactorily explained in terms of this velocity-modified temperature.