Oriented to the application of new generation strategic aircraft, M28 (Ti-4Al-5Mo-5V-6Cr-1Nb) is a novel metastable β titanium alloy with outstanding strength. A better understanding of its machinability is the cornerstone of the manufacture. In this work, milling experiments were made with the uncoated WC-Co carbide insert to investigate its machinability. Compared with milling Ti-6Al-4V, the cutting force is much higher and the tool life is considerably shorter in the milling of M28, especially in the high-speed cutting. Serious edge breaking is found at normal cutting speed. While a continuous band of flank wear with significant chip adhesion, which covers comb cracks in the cutting edge, is found in the high-speed cutting tool. The machinability of M28 is considerably poorer than that of Ti-6Al-4V. The hard-to-machine performance of M28 is considered to be subject to the material property of the metastable β titanium alloy as well as the competing mechanism among work hardening, strain rate hardening and the thermal softening in the cutting. According to the detection of the scanning electron microscope (SEM) and the energy dispersive spectrometer (EDS), the diffusion of C and Co generates a negative influence on the cutting edge and accelerates the tool wear.