The ternary transition metal carbide Mo_0.5W_0.5C was synthesized under high pressure and high temperature and the crystalline structure was confirmed by Rietveld refinements as being hexagonal (Pm2). The mechanical properties, bulk modulus and Vickers hardness were also investigated by in situ high-pressure X-ray diffraction and Vickers microhardness testing, respectively. The fitting bulk modulus of the ternary transition metal carbide is 399.9 ± 9.3 GPa, which is as compressible as diamond, and its asymptotic Vickers hardness is 15.3 GPa, nearly 60% harder than molybdenum carbide. The high bulk modulus is attributed to the high valence electron density and the greater hardness compared with γ-MoC is due to the strong bond between tungsten and carbon atoms.