Since difficult-to-machine materials are widely used in high-end equipment such as aerospace and marine components as well as precision molds, the development of high-performance cubic boron nitride (cBN) tools has been an important area of research. In this paper, we evaluate a new type of finer-grained, binderless end mill (binderless nano-polycrystalline cBN, BNNC) by comparing it to polycrystalline cBN (PcBN) end mills for high-speed machining of hardened steel. First, the process of tool preparation based on the above novel cBN is described and high-speed milling experiments were designed. Then, the cutting performance of the BNNC and PcBN tools during high-speed milling is analyzed and compared in four aspects: the cutting force coefficients, the friction coefficient of the rake face, the milling temperature and the tool wear. In addition, the wear mechanism of the two types of end mills was analyzed by energy dispersive X-ray (EDX). Afterwards, the trend of cutting temperature was further investigated by 2D finite element simulation. Finally, grey correlation analysis was applied to describe the correlation between the above factors and tool wear. This study shows that compared with PcBN end mills, BNNC end mills have longer tool life under the same cutting condition and are more reliable at high temperatures, which exhibit great application potential.