Abstract: In this paper the temperature distribution on the tool in milling of difficult-to-cut materials has been tested by the method of metallographic structure with the tonl of high speed steel. And non-steady-state temperature fields in milling has been calculated with the finite difference method. The experiment and calculated results are in good agreement. It is revealed that on the tool flank there exists a high temperature region with the shap similar to that of the crater on the tool rake face. It is the main cause of the flank wear in milling difficult-to-cut materials.