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