Material removal mechanism depends on the material property and machining parameters during machining process. This paper investigates the brittle removal mechanism of ductile materials with ultrahigh-speed machining. Based on the theory of stress wave propagation, the prediction model of critical cutting speed for ultrahigh-speed machining is proposed. The predicted critical cutting speed values are then validated with ultrahigh-speed machining experiments of Inconel 718 and 7050-T7451 aluminum alloy at the cutting speeds range from 50 m/min to 8000 m/min. The experimental results show that fragmented chips are produced above the critical cutting speed of 7000 m/min for Inconel 718 and 5000 m/min for 7050-T7451 aluminum alloy. The scanning electron microscopy (SEM) micrographs of fragmented chip fracture surface and finished workpiece surface are analyzed. Large amounts of cleavage steps and brittle cracks are observed on the fragmented chip surface. With the brittle cracks remains, the finished surface quality of ultrahigh-speed machining is worse than that of high-speed machining. The results show that the material property undergoes ductile-to-brittle transition so the brittle regime machining of ductile materials can be implemented with ultrahigh-speed machining. Taking both the machining efficiency and machining quality into account, the ultrahigh-speed machining is recommended to apply in rough machining or semifinishing, while high-speed machining is recommended to apply in finishing process. In the end, the definition and essence of ultrahigh-speed machining are concluded. This paper is enticing from both the engineering and the analytical perspectives aimed at revealing the mechanism of ultrahigh-speed machining and optimizing the machining parameters.