Abstract: The bending strength and abrasion resistance of diamond layer of Si-Ti-B doped diamond/cemented carbide composites depend on both the diamond particle size and the dopant content. The relationships between the mechanical properties and the diamond particle size, dopant content are considered as the relationships between properties and the mean free path. A maxi-mun was found in the bending strength (or abrasion resistance) vs. mean free path plot. The bending strength decreases with decreasing mean free path (binder layer thickness) to the direct contact of diamond particles. The bending strength decreases with increasing binder layer thickness is attributed to that the strength and hardness of binder (SiC, TiC) are lower than that of diamond. The heat-resistance of Si-Ti-B doped diamond is higher than that of cobalt bonded diamond.