Metal-bonded diamond particle composite blocks were fabricated by pressureless sintering, using multi-phase copper based alloys as the bonding materials. The processing sequences included ball-milling the diamond particles and metallic powders, uniaxially pressing the milled powder mixtures into green compacts, and sintering the green compacts in vacuum. The bonding materials, constituted of Cu, Sn, Ti, Mo and TiC, were prepared by blending various elemental and prealloyed powders. Addition of Ti as an active element effectively enhanced the interfacial cohesion strength, by developing an intermediate layer between the diamond particles and the matrix phase. This resulted in the observation that the failure mode of the composite blocks in a bending test was predominantly cleavage of diamond particles instead of pull-out of diamond particles.