Barry and ByrneT also suggested that the chemical wear was the dominant mechanism of CBN tools [9]. The works by Chou and Evans [10] and Poulachon et al. [11] indicated that the Tsize of carbide particles of the workpiece had significant effects on tool wear. TChou et al. noted that there was a transferred layer formed on the flank wear land, and it would lead to adhesive wear of the cutting tool [12]. All these studies provide more insight into CBN tool wear in cutting hardened steels. Nevertheless, the real mechanism governing the aforementioned peculiar increas- ing then decreasing tool life characteristic has not been clarified. Hence, the purposes of this paper are to study the cutting behav- iors and tool wear mechanisms in the turning of high hardness alloy steels by CBN tools under various cutting speeds.