Several sorts of cracks which appear in the carbide face-milling cutter, including the thermal crack, are investigated in respect with their significances as the causes of tool failures which occur over a wide range of the cutting speed. Tool failures at a high cutting speed are classified into several types from their appearances and each type concerns with a singular crack. The so-called thermal crack is one of these cracks which cause the tool failures at a high cutting speed. The cutting speed, the feed and the cutting fluid influence the sort of the crack which causes the tool failure so that the type of the failure changes. Including the thermal crack, some cracks are explainable that they are due to the fatigue of the carbide at an elevated temperature. But there are a few of such cracks that do not conform to the above mentioned explanation.