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 increasing 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.