The surface modification of multi-walled carbon nanotubes (MWCNTs) was carried out using plasma treatment. The microstructures of the prepared cermets with different additions of MWCNTs were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD). Mechanical properties such as transverse rupture strength (TRS), fracture toughness (K_IC), and hardness (HRA) were measured. The results showed that some reactive groups were successfully modified on the surface of MWCNTs, resulting in the improvement of dispersibility. Most of the amorphous carbons and impurities were peeled from the MWCNTs. Increasing MWCNT addition decreased the dissolution of tungsten, titanium, and molybdenum in the binder phase. The cermet with 0.5 wt% MWCNT addition showed the highest TRS and fracture toughness. The strengthening mechanisms were attributed to the finer grain size, the homogeneous microstructure, and the higher volume fraction of binder phase in the binder. The toughening mechanisms were characterized by bridging and pulling-out.