This paper presents a fundamental research which has been carried out regarding to thermal cracks of cermet cutting tools in high speed milling. This approach is based on a concept that the thermal cracks are easy to observe and have a good reproducibility when the tool wear is small; and to clarify the mechanism of cracking will be valuable for studying the problem of brittle fracture of cutting tools. It is shown that extreme growth of the thermal cracks causes the fracture of the cutting tools by itself under the condition of the thermal cracking predominates. The process of the growth of the cracks is examined using various tool materials. It is proven that the principal cause of the thermal cracking is the cooling of the cutting tool during uncut period of milling. The micro-fractography of the thermal cracks shows the nature of the path of the cracking, and it is found that the cracking is strongly influenced by the plastic deformation of binding metals. Finally, the thermal stress in the cutting tools is calculated under simplified model and it shows a good agreement with the result of cutting test and fatigue test of tool materials.