Abstract: Diamond particles,with average particle sizes of 12μm, 25μm and 50μm, were mixed with WC-10wt%Co in a 20% volume fraction and densified by using SPS method. Before mixing, the diamond was coated by nanometer sized SiC layer against oxidation and graphitization. The relative density of all composites reached 98%. The Vickers hardness did not differ so much from the matrix hardness of WC-10 wt%Co by the dispersing diamond. However, the indentation toughness remarkably increased to 17.8MPa·m^1/2 with 50 μm diamond dispersion. Crack deflection and block were clearly detected around diamond particles. The variation of diamond particle size did not affect the mechanical properties of the composite too much.