In order to achieve high-efficiency and high-precision machining of difficult-to-cut materials like Ni-based heat resistant alloy, it is important to suppress the flank wear and notched wear which occurs in the boundary region of the side cutting edge. This paper describes the damage morphology of polycrystalline cubic boron nitride (PCBN) tools in association with cutting speed and microstructure of Inconel 718. In the high speed turning of fine-grained workpiece in which flank wear is the main factor of tool life, DS1005 has long tool-life compared with conventional PCBN. In case of coarse-grained workpiece, the tool life is mainly determined by the notched wear. IT105 is able to suppress the notched wear, because of the advantage in terms of its mechanical properties.